1. A grouping of animals that share similar features. (e.g. cold-blooded, scales etc)
2. A race of 12-foot extra-dimensional beings that a few harmless crackpots like Icke believe rule the world.
Actually if you'd read up on David Icke, you'd see that when he says lizards he really does mean 12-foot reptiles from the fourth dimension. (Which doesn't mean he doesn't harbour some bordeline anti-semitic beliefs, just that the lizards aren't one of them). The guys a harmless maniac, not the next Hitler.