Toddler is a common term for a child between the ages of one and three years old, although some may consider a toddler as to be between two and five. During this period, the child learns a great deal about social roles and develops motor skills; to toddle is to walk unsteadily. The term cruising is used for toddlers who can not toddle but must hold onto something while walking.

The toddler developmental timeline shows what an average toddler can do at what age. Times vary greatly from child to child. It is not uncommon for some toddlers to master certain skills (such as walking) well before other skills (like talking). Even close siblings can vary greatly in the time taken to achieve each key milestone.

This is the age when they are known as, "the terrible two's", because of the temper tantrums they are famous for. This stage can begin as early as nine months old depending on the child and environment. The toddler is discovering that they are a separate being from their mother or caregiver and are testing their boundaries in learning the way the world around them works. This time between the ages of two and five when they are reaching for independence repeats itself during adolescence. Thus it is very important for the caregiver to be consistent with boundaries and discipline for the child’s safety and the caregiver's sanity through puberty.

Most children are toilet trained while they are toddlers. In most Western countries, toilet training starts as early as 18 months for some while others are not ready to begin toilet training until they are three.

 
From : en.wikipedia.org