Condor is unlike most other batch systems. The others typically involve submitting a job to one of several pre-defined job queues. These environments tend to become both constrained and complicated, as system administrators add more queues in response to the variety of user needs. Likewise, the user is asked to make compromises and is given the burden of both keeping track of which queues have which properties and deciding which queue would be the optimal one to use. Condor simplifies matters by acting as a matchmaker of ClassAds.
Sellers advertise specifics about what they have to sell, hoping to attract a buyer.
Buyers may advertise specifics about what they wish to purchase. Both buyers and sellers may have constraints that need to be satisfied. For instance, perhaps the buyer is not willing to spend more than X dollars, and the seller requires to receive a minimum of Y dollars. Furthermore, both want to rank requests to their own advantage. In Condor, users submitting jobs can be thought of as buyers of compute resources and machine owners are sellers.