What does an erase and sync delete on an iphone 4?

I connected my iphone to my brothers computer because i want his music but it asks me to erase and sync if i click on this will everything get deleted from my iphone 4 like my camera roll my old music my contacts and my email?

Essentially what happens when you "erase and sync" is everything that you are downloading will get erased.

Example: I have an iPhone that has music on it already. I decide I like my brothers playlist and decide to download his music on iTunes. All my music will be deleted and replaced by my brothers music.

This applys to only music! Unless you are downloading other things like pictures etc.