They essentially cover the function of English this/that, or sometimes just the, in English. Which form you use (i.e. whether it's ce, cet or cette) depends on the gender of the noun, and on whether the following word begins with a vowel or not.
- a masculine word will usually use ce: ce vin = this wine, but uses cet before a word beginning with a vowel: cet oiseau = this bird; cet autre vin = this other wine;
- a feminine word will use cette: cette fille = this girl.
When ce/cet/cette etc are used with a "pointing" function (i.e. where in English you'd use this or that, but wouldn't tend to use the), it's also common to put là after the noun phrase in French. For example:
ce livre-là this/that book
cette fille-là this/that girl
ce livre anglais-là this/that English book
The plural of all of these forms is ces, which therefore carries the notion of English these/those.
In spoken French at least, people don't tend to make much of a distinction between the notions of this and that. In formal or written French, you can make the distinction by using -ci instrad of -là: using -ci carries the notion of "nearness", like English this.