This site may have been posted here before; if so, I apologize to the post-er for not remembering. I do not know the meaning of the 2 labels on the left: Centum, and Satem Languages. Could someone help me out, please?
http://www.georgetown.edu/faculty/ballc/oe/oe-ie.html
Holy cow--if anybody's interested in a discussion board where you write in Old English, follow the links! Yikes.

Dear Jackie: I haven't had time to read the article carefully, but apparently it is a way of
classifying langueages by the word used to mean " a hundred". Here's a URL that seems
to give more information:
http://popgen.well.ox.ac.uk/eurasia/htdocs/anderson.html

Dr Bill, as usual, has come up with an excellent reference, if a bit technical. Don't go into it with the mistaken notion that the initial sounds of satem and centum are the same so what's all the fuss about. It is an ironic coincidence that the k sound in the centum languages has shifted to an s sound before the e in some languages.