There are a lot of different kinds of African masks, depending on what group of people made them. My favorites are from the Edo in Benin (http://www.metmuseum.org/toah/ho/08/sfg/ho_1978.412.323.htm).

Here are a few links that might help you, because there are too many to try and describe here:

http://www.rebirth.co.za/african_mask_basic%20forms.htm
Be sure to click the links at the bottom to learn about the different culture groups.

http://cti.itc.virginia.edu/~bcr/African_Mask.html
http://artchive.com/artchive/A/african.html
http://www.virginia.edu/artmuseum/VirtualExhibitions/african/intro.html
http://www.africa.upenn.edu/Face_Masks/menu_Face_Mask.html

Hope this helps!