Banding is an ability that modifies the rules for declaring attackers, declaring blockers, and assigning combat damage. The full rules for banding are described in section 502.10 in the Comprehensive Rulebook, but the basic premises are as follows:
When attacking, the attacking player may have attacking creatures form “bands”. Each band can contain any number of creatures with banding, and up to one creature without banding. A band attacks “together” - in other words, if a creature blocks one member of the band, that creature will automatically block every other member, too - even if it couldn´t usually block them. ( For instance: suppose your opponent controls a 3/3, and you have a pair of 2/2s. By attacking with the 2/2s banded together, you can guarantee that if the 3/3 blocks, it will die.)
The other part of banding is similar to the card Defensive Formation. When combat damage is being assigned, if it could be assigned to a creature with banding, then that creature´s controller gets to choose how the damage is distributed. ( This applies any time combat damage is being distributed. For example: an attacking band gets blocked; or an attacker gets blocked by several creatures, one of which has banding; or Ironfist Crusher blocks several creatures at once, one of which has banding; or even when an attacker with trample gets blocked by a single creature with banding.)
from " Saturday School Searchable Rules Database", www.wizards.com/