Do I need a carrier if my iphone 3g is jailbroken?

1. My friend is jailbreaking my iphone 3g and i was wondering if i need a carrier like verizon, at&t, etc to use it?
2. If i don't, then what are the features i can use? Like can i text and call people?

-thank you!

You will still need to have a mobile phone carrier to call & text. You can now use non-apple APPS. Jailbreaking is "rooting" not magic.