Sure - Wikipedia says it better than I could hope to:

As English-linguist Larry Andrews describes it, descriptive grammar is the linguistic approach which studies what a language is like, as opposed to prescriptive, which declares what a language should be like.[11]: 25  In other words, descriptive grammarians focus analysis on how all kinds of people in all sorts of environments, usually in more casual, everyday settings, communicate, whereas prescriptive grammarians focus on the grammatical rules and structures predetermined by linguistic registers and figures of power. An example that Andrews uses in his book is fewer than vs less than.[11]: 26  A descriptive grammarian would state that both statements are equally valid, as long as the meaning behind the statement can be understood. A prescriptive grammarian would analyze the rules and conventions behind both statements to determine which statement is correct or otherwise preferable. Andrews also believes that, although most linguists would be descriptive grammarians, most public school teachers tend to be prescriptive.[11]: 26