Per can also mean "according to" or "in accordance with." For example: -I completed the project per your email. -He assembled the furniture per the assembly instructions.

'Per' is used to describe ratios. For example, km/h is pronounced "kilometres per hour". m/s is "metres per second". I think that's pretty self-explanatory. 'By' and 'for' can mean lots of things and they both are in a lot of phrasal verbs. Here is something I typed up about their general meanings: http://www.italki.com/question/204803 Be careful of their exceptions.