The torrent of words your child has been hearing since birth is beginning to work its magic. By now, your baby's understanding of words far outpaces her ability to use them, and her babbling is probably starting to sound more like real words, including "ma" and "da." (Don't get too excited just yet -- they're more than likely not actual words, just a couple of the host of sounds she babbles constantly.)
At this stage your child still comprehends more from your tone than from your actual words. The more you talk to her -- while preparing dinner, driving, or getting dressed -- the more your baby learns about communication. In fact, one study found that the greatest predictor of later intelligence is how many words a child hears daily. Of course, idle background chatter and time spent parked in front of the TV don't count -- to help your baby's comprehension, she must hear words and language used interactively.
At nine months, a baby begins to understand the word "no," but she may not obey just yet. However, she will respond to her name by looking around or by stopping what she's doing to see who called. Reinforce her recognition behaviour by saying her name frequently.