They are about as related as a wolf and a poodle. They can be bred together, but who would do it? You'd be left with a mean little ugly dog. :P
Endlers are slightly smaller, and often have more color on the body and less on the tail.
Guppies tend to have less body color and brighter tails
In addition, endlers are a bit smaller, and the females have a more golden sheen to their scales, where female guppies are a bit more silver-gray.
Many endlers you buy at pet stores and such will have some guppy DNA in them due to their willingness to crossbreed.