For every player on your team, there is a present under your opponent's tree. You are a probe, and all presents are powerups. You must get every present under your own tree.
There are eight types of gifts, and they must be under your tree to be activated:
"Obviously a CD" - Data Disk, Ability to make Dragoons and Robotics Facilities
"Bling Bling" - Khalis Crystal, Gives you a Templar Archives
"Pointy Present" - Uraj Crystal, Tank / Ultralisk
"Neatly Wrapped Present" - Protoss Gas, Upgrades
"Poorly Wrapped Present" - Zerg Gas, Increased mineral gain
"Christmas Dinner" - Young Chrysalis, Vulture / Hydralisk
"PS 394" - Psi Emitter, Marine / Zergling
"Toy Robot" - Khaydarin Crystal, Support Bay (Reavers)
You can build cannons, gateways, battery shields, and robotics facilities. From gateways, the four Protoss ground units, and Reavers from the Robotics Facility. Both Archons are playable. Each team gets a Dropship / Overlord to use to spawn Marines, Vultures, and Tanks for the Terrans, and Zerglings, Hydralisks, and Ultralisks for the Zerg.
And Pylons, of course, blow up everything to the left and right, and above and below where they were created. This kills all Probes, troops, and buildings in their path.
None.