Strictly, a clitic is either an enclitic, where the clitic is with the preceding word, or a proclitic, which is with the following word. For example, in French celui-ci, ci is enclitic. In Spanish hacerlo, lo is enclitic. However, sometimes the word "enclitic" is used as a synonym for clitics in general.
Although clitics are usually defined in terms of pronunciation, they are often reflected in writing as well, with the two words being written together without space in between.
Some clitics can be traced back to Proto-Indo-European: for example, -kwe is the original form of Latin -que, Greek te, and Sanskrit -ca. This word means "and" and is said after the word being added, e.g. Senatus Populusque Romanus "Senate and People of Rome".
Clitics in various languages: