How do I know what is compatible with what?

This is a very, very broad topic.

EDIT: I thought about it, and to tell the truth, there isn't any one majestic chart I can think of that could list out every possibility. What I can try to give you is a descision-making process that will get you to where you need to go.

If you already know all of what I am about to type, feel free to bitchslap me for belittling you.

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Note: This process relies on the fact that you always keep your eyes and ears open. Always be reading reviews, gathering opinions from forums like this, examining what you hear about as you move through the process.

[code:1:87c7b0d142]Determine what you will primarily do with the computer.[/code:1:87c7b0d142]
The most important step in the entire process - this focuses your priorites, allowing you to invest extra money where you will see a return and saving money in areas where extra investment will not yield a noticable return. Say you want to game with this computer. You should focus mainly on the video card, CPU, and memory. This is also where you should determine what features you want in your motherboard - WiFi, Firewire, USB, eSATA, etc.

[code:1:87c7b0d142]Choose a CPU company - AMD or Intel?[/code:1:87c7b0d142]
This step will split you down the middle as far as your choices go. Obviously Intel chips don't work in AMD boards, and vice-versa. Move around and ask questions about whose processors are best at what. For example, the Intel Core 2 Duo is pretty much superior in everything right now and is an excellent choice for a jack-of-all-trades computer.

[code:1:87c7b0d142]Choose a line of processors (AMD Athlon 64 X2, Intel Core 2 Duo, etc).[/code:1:87c7b0d142]
This is an extension of the previous step, as certain lines of processors are better with some tasks than others. Note that multi-core CPUs are superceding single-cores as the mainstream products. Once again, the Intel Core 2 Duo is a prime choice for a system, while the X2 is still a viable choice for a dual-core, especially if your budget is below $150.

[code:1:87c7b0d142]Choose your socket (AM2, Socket T, Socket 939, etc).[/code:1:87c7b0d142]
This step as of right now is only crucial for AMD builders; if one builds Intel, Socket 775 (T) is the only real option right now. The socket determines the memory type and graphics interface, among other things.

[code:1:87c7b0d142]Scope out chipsets that are compatible with your processor.[/code:1:87c7b0d142]
This is a more vital step if you choose an Intel system rather than an AMD system. Not all chipsets work with all processors - for example, the Core 2 Duo only works with the Intel 965 and 975 series chipsets (There's also the nForce 5XX series and the ATi RD600; those are different stories). This is an area where AMD has an advantage over Intel - they've done a better job maintaining foward-compatability with their chipsets than Intel has.

NOTE: Fore Core 2 Duo, pay attention the the voltage regulator module, the circuit that controls electrical flow to the CPU. The current standard is Version 11.0 - earlier specifications will not work with Core 2 Duo.

[code:1:87c7b0d142]Begin searching for motherboards, taking into account what you determined in the previous steps.[/code:1:87c7b0d142]
This is where your search broadens - chossing a motherboard is difficult due to the sheer amount of choices. This is where reading reviews comes into play. Begin determining a list of reputable companies to look at. For example, ASUS has a good reputation for building solid Intel based boards. On the other hand, companies like Soyo, Biostar, and PC Chips have less than optimal reputations. Pay attention to what companies have good customer service, RMA policy, etc. Look at boards that have the correct socket, chipset, features, and CPU support from reputable manufacturers. You should end up with a moderately-sized, tidy list. Pay attention to reviews and problems with the board (All boards have problems. No board is perfect.), and you should be able to make a choice.

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Wow, that was long.

You could try going to pricewatch, pick motherboard and then the socket you want.

This is socket 775 start page for example
http://www.pricewatch.com/motherboards/10734-1.htm
They also have by specific cpu also. Example Home > Motherboards > athlon 64 X2 AM2
http://www.pricewatch.com/motherboards/992515-1.htm

A good start is the asus website.
They list their motherboards by cpu socket type ( which immediately differentiates intel or AMD and also age of processor) then by chipset (which is basically a performance vs. budget choice).

Hes only gonna see Asus MB that way though.
If you really wanna research different MB's the way I showed you will do that broken down by socket type. So choose the maker either intel/Amd then the platform/socket you want and you can see them broken down by price.

As far as I know there is no list such as you are asking for.

Thanks for all the replies - the reason I ask is because I am trying out for a job at a computer hardware store, and they expect me to know this shit.

I will keep cramming until my first shift starts, lol. If you have any additional information to tell me, please, let me know  

Thanks again!

So your trying to cram what takes years of know how and hands on into a week or less ?
Good Luck  

Cram computer info for a job? Thats a first. You'll only forget what you cram after you're done through.
Good luck anyways.

It's sales... I'm not building any systems or configuring BIOSs or anything like that. I need to know when somebody buys an ASUS motherboard that is S775 compatible if they can use OCZ memory or whatever. I need to know which mobos are problematic, etc.

Thank God for Wikipedia.

Amen to that Brother M.
(And the choir sings)