http://www.comdata.com/comdata/index.jsp
Comdata is a company that provides money management services to other companies.
In the transportation industry Comdata can provide services from fuel purchases to driver payroll. Depending on the service the Comdata card works like a standard credit card or a pre-loaded credit card/account.
Comchecks are simply paper checks used to access funds in the Comdata account, just like a personal check is used to access funds in a personal bank account.
Now if you are talking about being paid on your comdata card/fuel card I personally do not think it is worth it if your company also offers direct deposit. You will have fees if you want to access cash from your account, many places who are unfamiliar with Comdata/Comchecks will not accept them as payment. Everytime you want to write a check to access your money you have to call into Comdata and register the check you are using, etc.
IMHO, it is just not worth it for a company driver. Get yourself a bank account and have the funds transferred there and then get yourself a debit card.