i don't know, probably a cyst.
http://en.wikipedia.org/wiki/Cyst
http://www.nlm.nih.gov/medlineplus/ency/article/001639.htm
you might want to ask here:
http://curezone.com/forums/f.asp?f=333
if you don't get an answer, try here:
http://www.doctorslounge.com/dermatology/forums/viewforum.php?f=33