A french braid is usually considered one of the simplest of the different braid types. It starts with a basic three-strand (English) braid, using part of the hair, then gradually adds in hair to the strands after each cross.
French braids can be done on nearly any type or length of hair. The rule of thumb I use is that if a person's hair can be pulled back into a ponytail of any length, it can be put into a french braid. For shorter or finer hair, if a basic braid does not stay well, classic (two) braids will probably work better.
For instructions on how to do a french braid, see the DreamweaverBraiding site here.
For more pictures of french braids I've done, see the Flickr set here.