Anna is a prime example of this weekly update sent to my inbox from babycenter
"Ball," "dog," and "bye" may be your child's favorite — and only — words at this point, though some early talkers may have as many as 20 words in their vocabulary. Maybe she uses the words at the right time for the right things, or maybe she says "dog" when she sees other furry animals with tails. Don't be discouraged — that's an indication that she's learning to group things together. Just say the correct word in a matter-of-fact tone ("That's a horse, honey; horses are bigger than dogs"), and she'll soon pick it up.
Anna actually has a few more words - including moo (for cow and horse), baa (for sheep), uh-oh, bebe (baby), ma (for both mom and dad now), raaa (for lion).