Each ethernet device is assigned a unique hardware address by the manufacturer. Manufacturers are each assigned a range of addresses that they may use in their manufacturing, so that it is not possible for two vendors to assign the same address. These hardware addresses are called MAC Addresses.
You can search by MAC number to find out what vendor's hardware you are looking for. An interactive database is available from the IEEE for this purpose. You can also check Cavebear's list of Vendor Codes