In Starcraft, the Nydus Canal is one of the most valuable structures available to the Zerg race. It is also at the top of the resource tree, requiring the Cerebrate to morph his hatchery into a hive. Once it is available, though, it can be a devastating tool in the hands of the right Cerebrate.
The Nydus Canal is the only teleportation structure in the game; as such it requires both an entrance and an exit point. Once the entrance point is morphed on the Creep, the option to build an exit point becomes available - and this is where the beauty of it begins. See, the Zerg all share the same Creep; after all, they are all part of the same Swarm. As such, you can build your exit point right in the middle of the enemy Zerg player's base.
This requires a direct line of sight - ask your Terran friend to do a Comsat Sweep over the area, which is the easiest way. On a 3v3 map, you will want to place your Nydus Canal in the resource-gathering section of the enemy base; likely, the enemy's attention will be focused elsewhere. Once your Nydus Canal is fully functional, you can send whatever Zerg ground units you have.
Here is a list of what ground units can be sent through the Canal.
A couple of disadvantages exist with the Nydus Canal - should your enemy discover your little shenanigan growing on his Creep, he will take corrective action by destroying your Canal - when a Canal is destroyed, both ends are destroyed. Repeatedly ordering your units to enter a Canal will cause them to enter the Canal, pop out the exit, and make the journey back to the entrance canal