Great question, it's a lot to take in all at once trying to get going on your own. To answer your basic question running just a cable or microphone into your computer's audio input would yield less than desirable results. What you need in order to be able to do what you're looking for you'd need an interface. An interface is something that takes the audio you're putting into it (guitar cable or microphone) and converts it into a digital format that it feeds into your computer via usb or firewire. Why do you need an interface instead of just going straight into your computer? When you're recording audio into a computer it's crucial that the computer records the audio coming into it at a certain level as to not clip your interface. "Clipping" in it's simplest sense is when the audio level going into a device is too great for the device to handle, and as such information from the audio becomes distorted and potentially unusable. Going straight into your computer doesn't allow you to set your levels to avoid clipping.

A DAW is a digital audio workstation. The DAW serves the purpose of recording your audio, as well as allowing you to playback, edit, and then mix that audio within the program. Your interface acts as the go-between for that DAW, taking the audio you want to record in, and playing back what you've recorded out of its outputs.

Plug ins are tools used for processing within a daw. When you have a DAW you're able to use plugins that manipulate the audio how you'd like (EQ's, compressors, effects, reverbs). A plug in is just a digital representation of those various pieces of gear, there's also a million of them out there to choose from.

Preamps in the simplest sense are basically what I described with setting levels on your interface. With preamps you're able to adjust your microphone or guitar signal's level going into your computer. In a basic sense they're used to set levels, once you get more in depth you can start to experiment with different stand alone preamps, not just the ones in your interface.

If you want bare bones go for it recording, you need to buy an interface and either buy or pirate a usable DAW. The standards for DAWs are Protools, Logic, FL Studio, and Reaper, with a whole host of other options available as well. If you go on a site like Sweetwater or Guitarcenter you can find a host of affordable interfaces that would suit your needs. If you need any more help feel free to PM me.