The parts that you put on your bike are every bit as important as the actual bicycle frame. Unfortunately for the beginner you are pretty much at the mercy of your bike shop for their advice on parts. Fortunately most shop employees are pretty knowledgeable and honest about these things. On the other hand, manufacturers know what beginners typically look for and will often spec higher in those areas and skimp in equally important areas that are less visible.
So lets look at this in a few parts. First "The Group" ,"The Build Kit", the wheels, and the Fork.
The Group (a.k.a. the Groupo)
The group is the collection of components that are basically the moving pieces of your bike. They are grouped together because they typically all come from the same manufacturer and are all on the same or similar level of quality. Typically on the beginner level you are looking at a group from either Shimano or SRAM. Although you may see some Campagnolo or various other brands substituted here and there. I will focus on SRAM and Shimano because that is what you see most. Groups typically inglude: Front / Rear Derailleurs, Shifters, Brakes, Cranks, Bottom Brackets, Chains and sometimes hubs.
Here is a brief run down on each group
XTR- High end racing This is the cutting edge and very expensive.
XT "last years XTR" This is what most racers that are paying their own bills ride.
High quality, sometimes lighter than XTR, usually more durable than XTR, and more affordable than XTR
LX Racing to Sport level components
Deore Sport level components
Alivio / Accera Entry Level
STX Low entry Level
X.O - High end racing (SRAM Compatable)
X.9 - Affordable Racing (SRAM Compatable)
Rocket Affordable Racing (Shimano Compatable)
Attack Sport (Shimano Compatable)
SX5 Entry Level / Sport
7.0 Entry Level / Sport
3.0 / MRX Low end
Dura Ace High end Racing
Ultegra Quality Race ready Components (like XT)
105 - Great Value Personally this is my pick if you are getting into road riding
Sora Entry level, there are some differences in how these work, that make the jump to up to 105 well worth it.
Force High end Racing
You don't need to memorize all that, but I would write it down and keep the order in mind, as this is often the best way to compare bikes between brands. Most companies will spec a bike with a group that is appropriate for the frame. So if you see all XTR on a bike it is typically a nice frame, and if it is a mix of Alivio and Deore it is typically an entry level frame. The notable exception to this is Motobecan, which typically takes a very inexpensive frame and stocks high end parts so it looks like a good value, and if you just want the parts, it is a good value, but I recommend that you get a frame that matches your components.
You should also note that companies will often spec a mixture of these components. Usually they will put a relatively nice rear derailleur on and use lower componentry on the rest of the bike. They do this because the average rider is better able to notice a difference in performance in this component than any other. So when someone says that the bike is an "LX mix", it typically means that there is an LX rear derailleur and maybe a few other parts, and then a lot of Deore and maybe some Alivio. If they say that it has a "LX group" the entire group is probably LX although sometimes the Bottom bracket is down-speced because no one can see it.
The build Kit-
This part us usually forgotten or glossed over when you buy a bike, because to most people the only thing that matters here is if the seat is comfortable. The build kit is basically all the non moving parts, and they will come from a variety of manufacturers. Included in this group are: Saddle, Handle bars, Stem (connects Handlebar to Fork), Seat post, Grips, tires, etc.
Because of the wide variety of manufacturers it is impossible to write a general guideline for looking at your build kit. Fortunately, all of these parts are inexpensive enough that if you find you don't like them, you can change them out later. So for the most part, you don't need to worry about these.
Wheels - This is an area that most people don't know much about, so it is neglected in the buying process, but I suggest you ask a few questions about the wheels before you buy them. Wheels can be expensive to replace and maintain, and if you get wheels that are not up to the kind of abuse that you are going to give them, you will have to replace them quickly.
Here are some things you should know about your wheels before you buy them.
#1. Single-wall or Double walled rims? - Single walled rims in cross section look like a "U", they work, but they are not very strong. Double walled rims look like an upside down "A" in cross section. The extra "wall" makes them much more durable. So if you tend to go off curbs, do any mountain biking, or are buying a road bike, make sure you have a double walled rim.
#2. Steel or Aluminum Alloy -If your rim is a single walled rim there is a possibility that it is a Steel Rim which will be much wider and even less sturdy than an Aluminum rim. I would avoid Steel rims at all costs, you will be happier in the end.
#3 Freewheel or Cassette: Another thing that is difficult to see for the untrained eye, but is important in the durability of your wheel. This has to do with the cluster of gears on the rear wheel and how they are attached to the bike. On a freewheel the cluster of gears and the ratchet mechanism that allows you to coast without the cogs spinning, are all one piece. This whole mechanism screws onto a solid hub. The problem with this set up is that because the axle of the hub has to pass all the way through the freewheel before it attaches to the frame, there is a long section of unsupported axle between the bearings of the hub and the frame attachment point. This length allows more torque to be applied to the axle by the frame and makes it more likely that the axle will bend. That may be pretty hard to visualize, so let me say it this way. Freewheels = Bent Axles, Bent Axels lead to loose bearings, loose bearings lead to damaged bearings and hubs.
Cassettes are much more durable because they integrate the ratchet mechanism into the hub rather than the gear cluster. This allows the hubs bearings to be right out near the edge of the hub which reduces the stress on the axel and leads to a longer life for the hub.
In short if you plan on riding your bike with any kind of regularity you will want wheels that are with aluminum, double walled rims, and a hub with a cassette. While there is a range of qualities that you can get with these features, as long as you have those three things, you will have a wheel solid enough for any beginner.
There is no way to make a comprehensive list of forks here, but here are some basics.
For Road Bikes: You will have a choice between Aluminum, Steel or Carbon Fiber. Often the fork material will match the frame, but not always. Steel Forks are usually seen on steel frames. They tend to be a little heavier but have a little flex in them so they may ride a little more smoothly. Aluminum forks, usually found on aluminum frames, are light and rigid. They will be fast and responsive, but tend to transmit a lot of the vibration from the road into your hands. Carbon Fiber forks are found on bikes of almost any material. They are more expensive than the others, but they are light and absorb a lot of vibration so they make for a more comfortable ride.
Mountain Bike Forks: This is where is gets tricky because there are so many variations. Very few mountain bikes come stock without a suspension fork, and you will really only see rigid forks that were put on aftermarket by purists who typically only have one gear. As a beginner, there are probably only two things that you will look at, travel and brand.
Travel- Travel is how far a suspension fork (aka front shock) can compress. Remember that the longer the travel, the larger bumps the fork can absorb, but each increase in travel tends to increase the weight of the fork, and decrease the efficiency of the bike. 80mm of travel is what most cross country racers and recreational riders use. 100mm is still relatively efficient and a lot of sport riders like the feel of 100mm a little better 130mm and up are really designed for types of riding where you will either be going downhill most of the time or like to jump off of stuff. For most beginners, 80 to 110mm is fine.
Brand- There are a lot of brands out there and there is a lot of variation in quality within those brands, so while I shy away from the "Its RockShox it must be good" type of fork shopping, for the beginner that is not a bad way to start, and let the associate tell you about the particular fork you are looking at. Here is the rundown of some common forks.
RST- Typically entry level- the upgrade away from RST is usually worth it.
RockShox- Huge range of quality from pretty good to High end racing. Their high end forks tend to be light and flex a lot, but they have been making an effort to beef up some of the forks in their line.
Manatou Kind of the same quality as RockShox
Fox Probably the best forks on the market, they don't make any I don't like
Marzocchi - A wide range in this group as well, but in my experience they tend to have smoother travel and more lateral stiffness than the Manatous or RockShox. This is my second favorite brand.
There are a lot of others out there, but those are the common ones. The rule of thumb here for beginners is to look for 80-100mm of travel and remember that almost anything is an upgrade from RST.
So that's bike parts in a nutshell. Avid cyclists will note that I have neglected to mention a lot of stuff and have oversimplified some things, but beginners should have plenty to chew on. Good luck with selecting your bike.