These various pictures of fleas should give you a good idea of what fleas look like.
They are quite small, so these pictures are not representative of their actual size. They are about 1/8 of an inch when full grown and are a copper color.
They have a somewhat hard shell on them, so they do not smash or crush when hit. They also do not fly, they hop.
They are very fast, and when on a dog, will run through his fur while you are trying to bathe him.
They tend to stay in one area of your dog biting the same spots over and over again.
When they bite people, they tend to bite, hop, then bite again. You can tell human flea bites by the red marks that occur about every 1/2 inch apart.