A Job Type refers to the sort of construction job you're bidding, such as an Office Building, Residential, Hotel, etc.

Job Type is set on the Bid's Cover Sheet:

Create a new Job Type
Click Master > Job Types - the Job Type List (Master) dialog opens (this list can also be accessed from the Cover Sheet by typing in a Job Type that does not already exist in the database).
Click New - a blank line is added.
Enter the name of the new Job Type.
Click OK.
The Bids Tab can be Sorted and Filtered by Job Type. See Related articles for more information.
Remember, if you've entered similar Job Types and want to get rid of one, just use the global "Find and Replace" function in the Item Types list.