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During the first six months of life, the primary job that the baby is doing is to learn how to parse phonemes. We don't pronounce sounds precisely alike; the goal is to figure out when two sounds are slight changes in the same phoneme or are actually different phonemes. This isn't trivial. There are sounds which in one language are different but which in another language may be the same. At age 4 months, all babies appear to be able to differentiate any sounds used in any language. At age 6 months they only are able to differentiate the sounds which are different in the languages used around them. This was proved with a very clever experiment which took advantage of two words in one of the American Indian languages which to them were distinct but to me as an English speaker sounded exactly alike. A baby from an English-speaking environment sat on its mother's lap in a room while a recording played one of the words over and over again, with the other one being interspersed once in a while. Whenever the second word was played, a moment later a light would go on to the baby's right and a toy monkey would bang cymbals together for three or four seconds. So the baby learned to associate the second word with the toy monkey. Careful analysis of film taken of the experiment showed clearly that after a while a four-month-old baby would turn its head and look at the monkey after the word was spoken but before the light turned on and the monkey started making noise, which proved that the baby was reacting to the word and anticipating the toy. When the same experiment is done with a six-month-old baby, however, analysis of the film shows that the baby turns only after the light turns on and the monkey starts banging. By that age the baby is no longer reacting to the word; rather, it's reacting to the sound, because it is no longer capable of differentiating the words. Clearly somewhere in there that baby learned that those two sounds really weren't different and ceased to be able to differentiate them. This is an essential step; it means that the baby has gone from hearing sounds to hearing phonemes. The first level parse has begun to work. The second step is learning to make the sounds of the phonemes. This is done by "babbling". Now that the baby has the ability to hear and differentiate phonemes, it uses a closed loop to make sounds itself and listen to them to get them right. It will repeat them over and over, hence "ba-ba-ba-ba". This exercises the muscles and gets them into shape, and also reinforces the neural paths which control those muscles and helps make them more versatile. They've alreadly been exercises by nursing, but the controls neede for speech are much more complicated. And in the course of this, if it should happen to luck into creating a word, any adult nearby will instinctively perk up and repeat that word back to the baby. This is so deep an instinct in adults that most don't realize that they do it. And of course, certain sets of nonsense syllables have been seized on by adults and given meanings, especially "mama" and "dada" and "papa". The vowel sound in there is the easiest one a baby can make because both the tongue and lips are relaxed. The voice is going continuously and isn't modulating. Any other vowel sound requires use of muscles, and most other consonants require modulation of the vocal cords and possible control of the soft palate. "Mama" is a movement only of the lips and is the easiest sound to make. "Dada" involves the tongue and soft palate, "Papa" is a different movement of the lips but requires turning the vocal cords on and off. So these are among the very first sounds a baby will make, and they get assigned meanings to the baby by how adults react to them; the baby learns rapidly that "Mama" makes its mother perk up and smile, while "Dada" does the same to its father. (A side note: one of the earliest brain centers which activates in a baby is the ability to recognize faces; this appears to be working nearly at birth, and a baby can recognize a face nearly before it can recognize anything else. A baby learns to specifically recognize a small number of important adults very early, basically its care givers, and also can recognize smiles very early.) This feedback from adults turns out to be critical. A study was done on families from four very diverse groups (I recall that three of them were Chinese, American and Inuit but I don't recall the fourth) and they found a consistent behavior by adults during the interval when babies were learning language. Whenever a baby babbles a sound near to a real one in the local language, adults will echo it back; it becomes a game where the baby makes the sound and then the adult makes it back again. Of course, this amuses the baby (which amuses the adult) and encourages it to keep experimenting, while providing a valuable baseline to compare its own sounds against those made by someone already fluent in the language. It amounts to error checking. "You're making a sound close to an important one, but this is what it really should sound like." The child next has to understand what phoneme sequences are important and actually make words, and again adult behavior is critical. The baby is constantly listening to how adults speak to each other, but that's difficult because it's complex. When adults talk directly to babies, though, they use short sentences constructed with simple words and syntaces, they over-enunciate, spoken slowly and somewhat more loudly, and repeat several times: "baby-talk", in other words. This was found in all four of the cultures which were studied. Of course each time it will be said slightly differently, with the differences unimportant, and again this gives the baby valuable information about what is and is not actually information. The over-enunciation helps to delineate word boundaries far more than normal connected speech. Also, the simplified language eliminates most of the sophistication and complexity of adult speech and permits the baby to start learning the core parts of the language: short important words and simple grammatical constructions. All of which leads to the next stage: One-word sentences. The great leap is when the baby really begins to express itself by mastering a growing list of nouns. There are really two basic sentences that a baby uses heavily at this point: "Look, there's a ---!" and "I want ---!" and usually it isn't difficult to tell from context which the baby means when it uses a single word to fill in the space. Of those two, the former is far more important, because what it really represents is the process of accumulating a dictionary. First, the baby points at something and the adult will say its name, in baby-talk. Later the baby will start pointing at things and saying the words it thinks refer to them, and the adult will echo those words back, pronounced correctly. (More feedback.) Verbs come next (verbs are a harder concept), and often they too are used in single-word sentences, as imperatives. The final achievement is two word-sentences with a noun and a verb; that's the summit and from there it's all downhill. The baby begins to learn adjectives and adverbs after that and the sentence structures get more complex, and within another year the child will be expressing complex ideas and will actually be able to carry on a conversation. The surprising part of the studies which have been done on this is the extent to which babies rely on adult feedback in this process, and the extent to which adults are driven to provide that feedback. The best guess is that this is indeed instinctive in adults, one of the many behaviors towards babies which are ingrained in us. It's an interesting question as to whether this is genetic, however, or learned behavior. After all, all of us who have learned language have had this done to us, and we have seen it happen as we grow up later, since there are always babies around. A child raised without this feedback will never achieve the same proficiency in language that one given this feedback will attain. Would such a person as an adult still give that kind of feedback to a baby? There are many interesting aspects to this process. Phonemes are not the same everywhere. I don't pronounce my words the same way that someone from South Carolina does, let alone someone from London. If a British couple come to the US and have babies here and raise them, the kids will speak with the local accent and not with the accent of their parents. Evidently they're doing their phoneme-learning not just from listening to their parents, but also from everyone else around them and are correctly determining that their parents are anomalies. Another interesting fact about this process is that it can take place with two or more languages at once 
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