Classified Advertisements (classads) are the lingua franca of HTCondor. They are used for describing jobs, workstations, and other resources. They are exchanged by HTCondor processes to schedule jobs. They are logged to files for statistical and debugging purposes. They are used to enquire about current state of the system.

A classad is a mapping from attribute names to expressions. In the simplest cases, the expressions are simple constants (integer, floating point, or string). A classad is thus a form of property list. Attribute expressions can also be more complicated. There is a protocol for evaluating an attribute expression of a classad vis a vis another ad. For example, the expression "other.size > 3" in one ad evaluates to true if the other ad has an attribute named size and the value of that attribute is (or evaluates to) an integer greater than three. Two classads match if each ad has an attribute requirements that evaluates to true in the context of the other ad. Classad matching is used by the HTCondor central manager to determine the compatibility of jobs and workstations where they may be run.