Wow, hehe, very new to car audio <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'>
Anyhow:
1. Amps are commonly rated two ways, power and channels. It works by taking sound coming into it, boosting the signal and sending it on to speakers. This basically puts more power to the speakers giving louder music. Amps are measured in wattage output, most aftermarket speakers are between 40 and 100 watts, you want an amp at or below the speaker level (but not too far below, or you lose bass). Subs will typically be rated 100 watts and up to crazy ass numbers. Same rules apply here for choosing an amp. When looking at power, focus on the RMS rating, peak handling is nowhere near as important. The same here applies to speakers.. The number of channels is basically how many speakers it was designed to drive. Mono would be used for a sub, two channel for a pair of speakers (or bridged to a sub as a mono), and four channel to drive four speakers. It is fairly common to use a four channel amp to drive the front speakers and bridge to a sub in a budget setup. All in all, an external amp will put out more power and typically sound better than the amp built into your deck. Keep in mind this doesn't matter unless you are getting new speakers, further amping the stock speakers WILL blow them. In addition, if you get aftermarket speakers, you will want to find a way to bypass the factory amps, they cut the power coming into them to 5 watts for the factory speakers and will cripple the potential of your speakers.
2. A '92 fastback has 4x6 speakers all around. If you want something that wont require modification, stick with this size. If you don't mind modifying, you can fairly easily stick a 6.5" speaker up front by pulling out the OEM speaker baffle and replace it with some 1/4" mdf or plywood. The number of "ways" a speaker is rated is essentially the number of speakers. Factory is a single full range woofer. Two way (Coaxial) is a woofer and tweeter. Three way (triaxial) is typically a woofer, tweeter, and supertweeter. Four way is typically a woofer, midrange, tweeter, and supertweeter (quite uncommon). You will also come across component speakers, which will have a woofer and separate tweeter with a crossover. The crossover splits the high frequencies to the tweeters and the lows to the woofers. Components typically give the best sound, though will usually need to be powered by a bigger amp.
3. There's really only one type of subwoofer. The size, power, etc all depend on how you like your music. Smaller subs can hit fast beats better than big ones, but won't give you long drawn out booms. A 10" sub seems to be a happy median, but it all depends on the music you listen too. Higher power subs typically feed you louder and harder booms.
4. Battery doesn't mean anything unless it doesn't hold a charge anymore. Amps draw power directly from the batter, but the load all falls on the alternator to keep the battery charged. If you are running y our battery dead with your music, you need a more potent alternator. I honestly don't know what the stock alternator can handle, though I've run a 300 watt (RMS) system on it just fine.
That said, here's my system: Kenwood four channel head unit (22 watts per channel RMS), Infinity Kappa 6.5" components up front, Kappa 4x6 plate speakers in the rear, and a Sony 10" subwoofer running off of a 150 watt Kenwood amp. I primarily listen to rock, so my system works out fairly well. My next addition will be to amp my front speakers, as my current setup has no midbass (22 watts going to 90 watt speakers basically only gives the the high end notes <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'>).
To put on the 6.5" speakers, I needed to make a baffle board out of 1/4" plywood (as I described earlier). Definitely bypass the amps with aftermarket speakers, I noticed a big improvement in sound when I did on my rear speakers (fronts were installed bypassed.
Feel free to add or correct to my statements, and no there aren't any cliff notes.