Zerg Nydus Worm shown in the middle of the screen (that's the entrance, so is that it's @$$?):
This is what it looks like when it breaches the surface somewhere else:
The way it works is, it can breach anywhere there's creep. Overlords, after you've upgraded to a Lair (tier 2) can crap out Creep. So, you sneak to an enemies base with your Overlord, let it start crapping Creep, and make your Nydus Exit.
The results, very close to whatever mineral gatherers they have:
and when you're done and don't want to fight their guys, you just enter, and exit back at your base.
You can also use this by following your ally (if in multiplayer, in this case, I'm blue, ally is red, and we had teal and purple as enemies) with an Overlord while keeping all your stuff at your base. When he gets into trouble, start crapping Creep, and make your Nydus Worm Exit and deploy your units instantly.
He actually didn't need help here...those Colosi (Colossuses?), are Protoss AoE, other than Psionic Storm from a High Templar. Teal, the one saying "Not the Alamo!" only had basic units, and the AoE would've just torn through all of them.