What the heck is t8 and t12 bulbs?

So I got t5 lights on one of my tanks (Reef tank) and then on my 3 I just have the "regular" flo's. Id like to get....well I already have a t5 system coming but Id like to get a 14,000K light for one of my tanks but not sure what a t8 or t12 lights are? Id like to know the difference of a t8 and a t12