Like okaeri_man said so well, it's to do with formality and politeness. Always use "-masu" & "desu" unless you are with good friends (or the other examples above).
I'll just give some unsolicited advice as usual. Learning how to use the right "politeness level" in Japanese can seem like a hassle when you start. Why bother learning two forms for every verb? HOWEVER:
(1) It follows fairly simple rules, so it's not too hard to learn
(2) Once you get the hang of it, it makes Japanese much more FUN than other languages, because you can play around with it
(3) If you can speak at the right politeness level, you will get on much better in Japanese society - especially the world of employment.
Especially if you are a young woman, and if you are dealing with older men, you need to make a big effort to use the right politeness level (basically, be as polite as possible without going over the top). Japanese people will be more impressed with you, and they are more likely to trust you and accept you.