this year in my physics class we will be going onto relativity. but i cant be bothered waiting.

could you please explain to me what relativity is basically about. you dont need to go into much detail, but ive just never understood what it was really. my friend told me it was about energy change or something when something happens to something. i dunno, i cant even remember what he said now, because what he said was pretty vague.

if you want to go into more detail than a quick summary, that would even better, but no need really.