What gets erased when you erase iphone from icloud? Does your imessage get deleted?

What do you mean "erase from iCloud"? If you use the erase all content and settings feature on the phone, everything on the phone gets wiped out, but your iCloud account content is not affected until or unless you sync another device to that same iCloud account.

iMessages will be saved in your iCloud backup if you made one, and can be restored from that backup file.

if you are talking about a remote erase from the find my phone tab when logged in to your iCloud account, again, yes everything gets wiped from the phone. But again, if you have a backup file in iCloud you can restore from that backup file (or a backup file I. ITunes if you made one there).

If you have not backed up the device and erase it, yes all content is gone for good then.