first thing, use the search engine and check out the terms section.

a pre amp is designed to amplify the signal from the very low level produced by a microphone or instrument to line level.
it is necesary to have one.
often, there are pre amps in sound cards.
it's written in the specs of the card.

one distinction you need to understand is between instrument input (known as high-z or high impedence) and microphone input (low-z).
the crapy sound you're getting from the guitar is because you're conecting a high-z source to a low-z input, or even the line input ( that's incorrect)
the third type of input (and last for now) is line level input, which doesn't require a pre amp (it's already line level )
this includes cd, device line outputs etc.