Batch Apex gives you the ability to operate over large amounts of data by chunking the job into smaller parts, thereby keeping within the governor limits. Using batch Apex, you can build complex, long-running processes on the Force.com platform. For example, you could build an archiving solution that runs on a nightly basis, looking for records past a certain date and adding them to an archive. Or you could build a data cleansing operation that goes through all Accounts and Opportunities on a nightly basis and reassigns them if necessary, based on custom criteria.

All batch Apex jobs run asynchronously. After a job is placed in the job queue, it runs when system resources become available.