Clawhammer is a highly rhythmic class of a fingerpicking technique that is used on the banjo and very rarely on a guitar. Traditional picking styles include those for Folk, Bluegrass, and classical guitar and consist of an "up-picking" motion by the fingers and "down-picking" motion by the thumb. The main reason it's called Clawhammer is because of the shape of the hand when it's picking the banjo; the hand assumes a claw like shape and the strumming finger is kept stiff, striking the strings.

Some of the most modern musicians to use the clawhammer technique are Bob Carlin, Dwight Diller, Eric Clapton, Abigail Washburn, Dan Levenson and several thousand others. In fact even the actor Steve Martin has been caught using this style more than once as a part of his stage act in the 1970s.

Clawhammer and another technique called "frailing" are a common component to American old-time music. However even though Frailing and Clawhammer could be used interchangeably, a lot of old-time players draw a large distrinction between the two. On a banjo frailing usually refers to always picking the drone string, while clawhammer allows the picking of strings with the thumb also referred to as "drop thumbing." Some players go even further and put a distrinction between clawhammer and drop thumb as well.

There are several books as well as websites where you can find out more information on this type of banjo playing, how to play Clawhammer banjo, Clawhammer banjo tabs, as well as other information. If you're interested in this you might want to check out the following books and or sites:

Mel Bay Clawhammer Banjo from Scratch: a guide for the claw less: Category - Book

Clawhammer Banjo Volume 1: Category - DVD

Learn to Play Clawhammer Banjo, Lesson 1 & 2: Ctaegory - Book

Mel Bay Presetns Complete Clawhammer Banjo: Category Book

Clawhammer Banjo-Repertoire and Technique Vol 1: Category - Video

Wikipedia
ezFolk
Clawhammer
Clawhammer Banjo
NativeGround

Good luck on your search for more incredibly interesting information on this unique instrument! I'm sure you could also find some videos online as well if your just curious about the actual finger placement or examples of exactly what clawhammer banjo is by looking online. For myself personally, I understand some things better when I can visually see the topic at hand so something like this might be very helpful to you! Try sites like Google Video and YouTube. Have fun!