Condor works with grid resources, allowing users to effectively submit jobs, manage jobs, and have jobs execute on widely distributed machines. This is Condor-G.
The resources are machines. The machines are likely to be in multiple locations, and they are owned and administered by different groups. This would make use of these machines difficult for a single individual.
Condor uses Globus to provide underlying software needed to utilize grid resources, such as authentication, remote program execution and data transfer. Condor's capabilities when executing jobs on Globus resources have significantly increased. The same Condor tools that access local resources are now able to use the Globus protocols to access resources at multiple sites.
Condor-G is a program that manages both a queue of jobs and the resources from one or more sites where those jobs can execute. It communicates with these resources and transfers files to and from these resources using Globus mechanisms. (In particular, Condor-G uses the GRAM protocol for job submission, and it runs a local GASS server for file transfers).
It may seem like Condor-G is a simple replacement for the Globus toolkit's globusrun command. However, Condor-G allows you to submit many jobs at once and then to monitor those jobs with a convenient interface, receive notification when jobs complete or fail, and maintain your Globus credentials which may expire while a job is running. On top of this, Condor-G is a fault-tolerant system- if your machine crashes, you can still perform all of these functions when your machine returns to life.