Whenever a creature you control attacks, it gets +1/+0 until end of turn for each other attacking creature that shares a creature type with it.
4/1/2008 This ability counts creatures, not creature types. For example, if you attack with five creatures — an Elf Shaman, an Elf Warrior, a Goblin Shaman, an Elemental, and a creature with all creature types — the ability will trigger five times. Those creatures will get +3/+0, +2/+0, +2/+0, +1/+0, and +4/+0, respectively.
4/1/2008 In a Two-Headed Giant game, only creatures you control trigger the ability and get the bonus, but your teammate’s attacking creatures are included in the calculation of those bonuses.