The 18650 designation gives the size- 18mm diameter, 65 mm long (more or less - in practice, there is some variation). "0" indicates the battery is cylindrical. They are not compatible with AA's, but there are lithium-ions that are of that size. unless specifically stated,the AA light's electronics may not be compatible, however. The energy density is significantly higher, compared to other chemistries.

Check out candlepower forums. There is a ton of good info there, as well as relevant cautionary advice.