Banding is legendary as one of the early mechanics included in Magic (it was in Alpha,) but also one of the ones that was abandoned due to complexity to how it functioned. The concept was that some creatures could combine together in their blocking and give the advantage to the blocking player rather than the attacking player. Think Three Musketeers - "All for one, and one for all!"

702.19. Banding

• 702.19c. A player may declare as many attacking bands as he or she wants, but each creature may be a member of only one of them.
• 702.19d. Once an attacking band has been announced, it lasts for the rest of combat, even if something later removes the banding ability from one or more creatures. However, creatures in a band that are removed from combat are also removed from the band.
• 702.19e. If an attacking creature becomes blocked by a creature, each other creature in the same band as the attacking creature becomes blocked by that same blocking creature.
• 702.19f. Banding doesn't cause attacking creatures to share abilities, nor does it remove any abilities. The attacking creatures in a band are separate permanents.
• 702.19g. If one member of a band would become blocked due to an effect, the entire band becomes blocked.
• 702.19h. A player who controls an attacking creature with banding chooses how combat damage is assigned by creatures blocking that creature. A player who controls a blocking creature with banding chooses how combat damage is assigned by creatures it blocks. If the creature had banding when it attacked or blocked but the ability was removed before the combat damage step, damage is assigned normally.
• 702.19i. Multiple instances of banding on the same creature are redundant.

Rules Translation

• 702.19c. - You can have as many attacking bands as possible. Creatures can only be part of one band at a time.
• 702.19d. - Bands continue to exist through combat, even if the world ends, the band exists until combat ends. But if a creature leaves combat it's not in the band anymore.
• 702.19e. - All for one and one for all. If one creature is blocked, the band is blocked.
• 702.19f. - Bands are still a group of individuals. They don't merge in any way.
• 702.19g. - If one creature in the band gets blocked by some effect, the whole band is blocked by that effect.
• 702.19h. - The player who controls the band decides how combat damage is assigned to his creatures (reverse of usual when the one dealing the damage assigns how it is dealt.) If a creature loses banding, it's out of the band and thus dealt damage normally.
• 702.19i. - Banding does not stack. If a creature has banding twice, it can still only be part of one band.