While much of what you hear during this stage is random babbling, between 9 and 12 months babies begin to make more purposeful sounds. They're also becoming more aware of your response, so if your son says "Dada" and Dada himself howls with delight, your baby is likely to do it again. At the same time, his motor control is still developing, so he won't yet have a wide range of sounds, and many "words" may end up sounding alike. In our house, "Da" meant ball, doll, and dog, and only certain cues could help me decipher what my son, Nathan, was talking about.
However, if your baby doesn't have any words at this point, don't worry. Many kids -- especially boys -- wait until around 12 months to start talking.