A clitic is a morpheme that behaves syntactically like a word (i.e., is syntactically free) but is phonologically bound to another word. An example of a clitic in English is -'s as a contraction of is. A clitic may have a non-clitic alternant, in this case is.
Clitics versus affixes
A clitic differs from an affix in its degree of syntactic freedom. A clitic may attach to words of multiple syntactic categories or even phrases, while an affix attaches to a single word in a single syntactic category. For example, -'s as a contraction of is can attach to a phrase, e.g., "The Duke of Kent's going to arrive soon," while the verb ending -s cannot.
The possessive case ending -'s shows signs of having evolved from an affix into a clitic. For example, it can attach to a phrase, e.g., "the Duke of Kent's Waltz."