What type of wall sheathing is on my house?
It was made by Celotex as well as some other companies. It comes under the nickname Buffalo board and Beaver board depending on where you're at. I don't know that it's manufactured anymore, however I viewed the link from the previous answer and it appears it could be the same thing. 

If your house is older then it is probably the original buffalo board material. If it is newer it could very well be the exact same thing that is sold at Home Depot mentioned in the link that was provided in the first answer.