These are generic rules
based on a mass generalisation of a lot of different visions of the creatures
Dragons have wings, breathe fire and have four limbs. They look almost exactly the same as reptiles, but aren't, due to their uncanny ability to fly*. Even though dragons are thought to be green, they can come in all sorts of colours. Also, it is not unusual to find a dragon with magical powers. They tend to be about 20-30 metres in length at an adult size from head to tail. They sometimes have the ability to talk as well.
Wyverns also have wings, but don't breathe fire and only have two hind legs. They tend not to have magical powers, but some still do. They tend to be smaller than dragons, only amounting to about 10-12 metres in length from head to tail. Their tail is also much smaller, sometimes even non-existent. They tend to be found in shades of brown or green.
Some of this might be wrong. Correct me if it is.
* Though the idea of them being reptiles cannot be ruled out, as the Pteranodon was a reptile and also known to fly. (thanks cryokinetic!)