An Arena format that pits teams of 5 players against each others, hence the name.
5v5 Team can have up to 10 players on the roster.
5v5 teams are the most rewarding point wise of all format. However, due to the difficulty of organising so many players, 5v5 remains one of the less accessible format.
While many people, including Blizzard, have claimed the 5v5 format is the fairest and most balanced of the format, it has not received overall acceptance from the e-sport community and 5v5 tournament are actually quite rare.