Yo Harleyrider! You are getting true answers from experts who aren't always great teachers. Here goes:
1. A preamp is an amp that raises the weak signal from a mic to line level. That line level output can either be sent to a power amp, then speakers, or to a sound card or recorder. Your combo amp has a preamp in it, or it wouldn't have a mic (XLR) input. Preamps run from $10 per channel to $4500 per channel, or even more, and you get what you pay for. Better preamps usually mean better sound. Some are "clean"- they make signals stronger, but don't change them much. Some are more "colored"- they modify the sound, hopefully improving it. The pres in your amp are on the cheap side, as they go.
2. Phantom power is power that is sent back up the mic cable to a mic to power it. Some mics can provide their own power by battery. Tube mics have a tube in them, which requires more power than phantom power can provide, so they use a special multi-pin cable and a separate power supply.
Usually, dynamic mics don't require phantom power, but condenser mics do, excepting tube mics. There are a few exceptions to that rule. You can buy a phantom power supply to feed condensers to use them with gear that doesn't provide phantom power. Example- Rolls PB23, about $50.
3. Condenser mics are often more sensitive than dynamic mics, and usually have a much hotter output. They can be feedback prone because of their sensitivity. Usable ones can be had cheap- Examples- Studio Projects B-1 ($80) and Marshall MXL V67G ($80-100). Note that studio workhorses are $500-$1000, and top of the line srudio condensers are in the range of $2000-$8000! Ooh baby! Condensers most definitely can be used live, and often are, especially for instruments, but a good PA with good equalization and an engineer with a clue are very helpful here. Condensers are not usually used live for mic'ing amps, or for vocals, but they are often used on violin, flute, recorder, acoustic guitar, banjo, mandolin, etc.
OK, there's some basic answers, but... the info you've given gives very little clue what you are really trying to do. OK, let me guess- you're trying to plug something into this combo amp you have, but we don't really know what. I would say if your music is basically acoustic, and you use a lot of instruments, a condenser might come in handy. If you are basically electric, and you use mics for vocals or percussion, you'll prefer a dynamic for vocals, snare, toms, and kick, but you'll want condensers for overheads. For most live applications, it's hard to beat a Sennheiser MD421. It's a dynamic mic that runs about $200. It'll be a real workhorse for you.-Richie