A genus is a group of very similar species (the plural is genera). The practice of naming genera and species is called alpha taxonomy. When we name a species, we say both the genus and species names together. For Tyrannosaurus rex, “rex” is the actual species. “Tyrannosaurus” is a genus (notice that both words are italicized; genera and species names can also be underlined). This means that you could have a bunch of closely related species which are all grouped under Tyrannosaurus. In fact, there is a form from Mongolia called Tarbosaurus bataar, which lived just a few million years before Tyrannosaurus rex, and which is almost identical to it. Some paleontologists think should be considered a species of Tyrannosaurus: Tyrannosaurus bataar. It is acceptable to give the genus name as just the first letter followed by a period, for example T. rex (but not as T-rex, T-Rex, as it is sometimes given). So, we could say that there are two species of Tyrannosaurus: T. rex and T. bataar.

Unfortunately, we are even vaguer on how to recognize a genus as how to recognize a species. As with the morphological species concept, it is pretty much based on similarity. But how similar? The genus Tyrannosaurus belongs to a group of theropods (meat-eating dinosaurs) called tyrannosaur . . . More