Anyone knows what a battlegroup is?

if anyone knows please answer

A battlegroup is a group of realms (servers) that will be in the same battle grounds.  So there is only a set number of servers that enter your realms battlegrounds.  There is a list of what realms are in what groups on the WOW website.

I hope that makes sense.

Battlegroup. Servers with the same battlegroup meets in WSG/AB/AV/EOTS/Arenas

So, just to confirm, in a BG I could be playing people from multiple servers, not just my own?

If you mean Battleground, then thats a place where you go to fight against horde with a large raid. If you mean battlegroup, its the stuff about the servers liek the guy said before