By definition, a "toddler" is one who toddles or, in this case, a very small child who toddles. "Toddling", of course, is the word we use to describe the initial method that small children undertake in order to move from place to place. Therefore, a "toddler" is an infant who has begun to walk. This is the first developmental milestone of toddlerhood and usually occurs somewhere between 11 months and 15 months of age.
Some babies begin to say their first words as early as six months old; some barely speak at all until they reach their first birthday. Social interaction, however, has already commenced: babies say "mama" and "dada", wave bye-bye, roll the ball back and forth, pick up toys, and respond to their names.
When a child progresses to toddlerhood, he begins speaking in phrases or short sentences and can put social interaction together with the proper words. This usually begins and/or increases at about 12-14 months. Likewise, a toddler's coordination improves and often they will carry toys or objects and be more adept and picking them up and setting them down versus throwing and dropping.
Toddlers will also begin to show interest in more "adult" pursuits, such as bathroom habits, cooking, vacuuming, and so forth. Toys at this age often mimic these duties, and toddlers have fun as well with the adult-sized versions. A wise parent will take advantage of this and begin to teach her child these basic skills.
Potty-training, too, is another highlight for toddlers. If a child is allowed, around his first birthday, to become accustomed to a potty seat or chair and then, beginning around age 13-14 months, sat upon it several times daily at regular intervals, he should be trained and out of diapers long before he becomes a preschooler.
Older toddlers, from about 13-15 months, will try to "read" their storybooks and even engage in make-believe. They also become even more self-aware and start to realize that adults, and others, will exhibit certain reactions when they "perform". This sets the stage for the so-called "terrible twos".
The "terrible twos" actually begin at around 16 months and last until nearly age three. The aforementioned reactions are gauged and contemplated as parents and others respond to a child's words, actions, and tantrums. This is the second battle-testing ground in parenting, the first being those initial sleepless nights. Toddlers at this age are more daring, without the necessary common sense to balance this phase. Children begin to open doors and walk outside, or merely climb out of their cribs, or find dangerous objects that, prior to now, were ignored.
Talking and mobility increase, as a toddler in this stage discovers more physical pursuits, such as running, throwing, hitting, biting, pulling the cat's tail, and screaming for no apparent reason. The "terrible twos" are not meant to drive a parent to an early grave, but is simply a time when a toddler's world is quickly expanding and they are learning to deal with and live in that world; their communication skills have not quite caught up to their development, and they have certain ways to garner attention which may not be socially acceptable.
By age three, children are considered "preschoolers" and are ready to enter the world of formal education. Each child does indeed develop at a different pace, and there is no set cut-off age for any of these developmental milestones.