Can be used when referring to having a beer.
Also may be in reference to smoking weed.
Also less urbanly used in England to describe having a cup of tea.
The term "brew" can refer to a friend, much like brother, Man, dude and homie. This is not commonly used when referring to a person outside of their immediate presence, only when you are having a direct conversation with the person.
A keg of beer, or mini keg. Most likely a mini keg, usually one that can be held during a party.
Billy is holding a Mini keg of Heiniken.
Johnny: Hey billy can I have some of your Heiniken?
(reaches for mini keg)
Billy:(Piss Drunk) Bitch don't touch my Brew!