A string quartet is a group of four (4) musicians playing string instruments — usually two (2) violins, a cello and a viola.
This group may be hired to play during the wedding ceremony, beginning with the prelude until the wedding postlude. The string quartet may also be hired to play background music during the wedding cocktail reception.
Here's a great primer to understanding all about working with a wedding string quartet via juicyglitter.net.
Image via juicyglitter.net