Clitics are morphemes with the syntactic characteristics of words. However, they must be attached to another word. They have the form of affixes but the distribution of function words.
Clitics are often pronouns and auxiliary verbs. In the phrase my daughter's room, the 's of daughter's is classified as a clitic. It occurs after the host daughter.
In English clitics usually occur after the host. These clitics are classified as enclitics. Examples of enclitis are he's, they've, and we'll. English has few examples of clitics which occur before the host. However, one example is y'all, a word which is often used in the southern United States.
Clitics are different from affixes. Unlike affixes, clitics always occur at the edges of a word. For example, in the words it's, its and can't, the clitics occur word finally. However, in the word kingdoms, the derivational affix, which is -dom, occurs before the inflectional affix -s. The derivational affix isn't at the edge of the word kingdoms. This is different from the behaviour of clitics.
Bound morphemes with the form of affixes but the distribution of words are known as clitics. They always occur at the edges of a word and are unstressed. Clitics which attach after their host are enclitics while those which attach before their host are proclitics.