The typical handshake/greeting when two guys or "bros" are cool with each other meet. It can be spontaneous and is not limited to only old friends but also possible new bros whom are perceived as to being cool with.
The most generic Bro Shake consists of a mutual hand-grab, then a back-pat and usually ending with a fist pound.
Variations are usually created and if the two bros are very good friends, they may have a more intricate Bro Shake.
Bob: "Holy shit, is that you Sam?"
*Bro Shake Initiated*
Sam: "I haven't seen you since you piled that chick in college Bob!"
Hugh Hefner: "Welcome to the Playboy Mansion boys, the ladies are a plenty."
*Bros walking in, each Bro Shake/ing Hugh*
Stranger 1: Thanks Hugh!