Another noob with questions!

I just started to learn about Bitcoin Mining and want to get into at whilst its taking off. I'm 19 and have a decent PC for gaming but not sure how well its going to do in mining. I have a Twin Frozr 7870 and pull just under 400MHs using 50miner and 50BTC. I have just inder 0.02 BTC after a day and feel like I could be doing more. Anyway, my questions.

• 1 - What is mining? Like, why do I have to use my GPU? What is using my GPU?

• 2 - I heard that Slush&GUIminer would have been a better combination than 50miner and 50BTC, but I've made an account on the website and when I log in after I have activated it, it tells me the user and pass dont match. Any ideas?

• 3 - Im a bit of a noob with temperatures. So at the moment my GPU runs at 61c at full power when I'm mining. I have a few Noctua fans running in my PC but I'm not sure how long I can safely leave my PC on mining. So far it's been mining for about 30 hours and Im feeling uncomfortable about leaving it on that long. Am I safe?

• 4 - I dont actually pay the electric bill at home this will hopefully just be a way to make a little money on the side (internet), is it worth sticking too or should I buy another 7870? And will I make my money back on it?

Thanks guys.