In a nutshell, HTCondor is a specialized batch system for managing compute-intensive jobs. Like most batch systems, HTCondor provides a queuing mechanism, scheduling policy, priority scheme, and resource classifications. Users submit their compute jobs to HTCondor, HTCondor puts the jobs in a queue, runs them, and then informs the user as to the result.
Batch systems normally operate only with dedicated machines. Often termed compute servers, these dedicated machines are typically owned by one organization and dedicated to the sole purpose of running compute jobs. HTCondor can schedule jobs on dedicated machines. But unlike traditional batch systems, HTCondor is also designed to effectively utilize non-dedicated machines to run jobs. By being told to only run compute jobs on machines which are currently not being used (no keyboard activity, low load average, etc.), HTCondor can effectively harness otherwise idle machines throughout a pool of machines. This is important because often times the amount of compute power represented by the aggregate total of all the non-dedicated desktop workstations sitting on people's desks throughout the organization is far greater than the compute power of a dedicated central resource.
HTCondor has several unique capabilities at its disposal which are geared toward effectively utilizing non-dedicated resources that are not owned or managed by a centralized resource. These include transparent process checkpoint and migration, remote system calls, and ClassAds. Read section 1.2 for a general discussion of these features before reading any further.