What is the difference between a credit union and a bank, and is my money safer at a credit union?

I know. I’m full of questions today.

I did some online research, but it all seems so technical. Can someone explain it a little better? Do credit unions have investments? Where do they get the money for loans and such? We were thinking of closing our bank account and joining the Cal teacher’s credit union. Good idea? It’s not like I have tons of money that I am worried about losing, I just want to know the differences.