Yes, fleas are parasites that suck blood from their host animal, whether that be a dog, cat, or another type of mammal.
Fleas need to eat blood because they require it in order to lay eggs and reproduce. When a flea pupa emerges from his silky cocoon, he has a very short time in order to find a blood meal or else he will die.
Once an adult flea does successfully obtained his first blood meal from a host, he can then go up to one year without feeding on blood again and still survive.
But adult fleas prefer to live on the body of their host animal, continually feeding so that they can rapidly reproduce and expand the flea population.