I can only answer your first one m8.

Compression:

A compressor is used to control the dynamic range of the audio you are recording. Vocals tend to have lots of peaks and dips, giving an uneven level, so a compressor is used to give a more frequent level to achieve more control over the audio.

Basically, instead of manually adjusting your channel fader up and down to get an even level you would add a compressor.

I hope this makes sense.

Thanks mate! It makes perfect sense.