In morphology and syntax, a clitic is a morpheme that is grammatically independent, but phonologically dependent on another word or phrase.[http://www.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsACliticGrammar.htm SIL Glossary of Linguistic Terms: What is a clitic?]
It is pronounced like an affix, but works at the phrase level.
Clitics may belong to any grammatical category, though they are commonly pronouns, determiners, or adpositions.
Note that orthography is not a good guide for identifying clitics: clitics may be written as independent words, bound affixes, or separated by ...