I hear alot of people using pound cake for carving. Is there a reason for this? Does it hold up better, or is it stronger? I have just started the carving. Doing my first actually - a tractor. Any tips for the tractor?

Secrets of cake carving:

http://www.cakecentral.com/cake-decorating-ftopict-59361-.html

3d tractor help thread:

http://www.cakecentral.com/cake-decorating-ftopict-623279-.html

http://www.cakecentral.com/cake-decorating-ftopict-610913-.html

Tractor cakes (from the galleries):

http://www.cakecentral.com/modules.php?name=gallery&meta=search&type=full&search=tractor&album=55&search_user=

All CC extended/enhanced cake mix recipes:
(Suitable for carving or 3d.)

http://www.cakecentral.com/cake-decorating-ftopict-614378-.html

HTH

I'm pretty positive that the links JanH provided will give you all the answers you need (she's a fountain of knowledge ) but wanted to say that any "dense/heavy" cake would be good for carving. That's why a lot of people use pound cake. Also many folks have more success with chilled or slightly frozen cakes when carving.