Everything Dean said was correct. I just want to add that people will say also 'cause' as slang for because. e.g. Why didn't you come to school yesterday? 'Cause I didn't want to. If people are using cus or cuz in txt then they mean this slang version of because. Both cus and cuz are used, though I think I've seen cus more frequently. The slang 'cause is pronounced as cus or cuz. I would say you should only use the slang with friends in informal situations.

Because is a conjunction, cause is the reason. For example I didn't go to work yesterday because I had a cold, so the cause is I had a cold. So if you say I didn't go to work yesterday cause I had a cold, people will understand, but it's not good saying it this way. If my understanding is wrong, please correct me.

The difference between "cause" and "because" is that the first word is more informal and the second word is more formal. You can use "because" in a meeting and "cause" with friends. ????