What is the difference between Queues and apex Flex Queues?

from Help & Training :

  "Normally, a Batch Job which has not started yet will assume the status of Queued. The batch which is currently being processed will be in the InProgress status. Batch jobs usually have the limit of five queued or active jobs simultaneously. With Flex Queues, any jobs that are submitted for execution but are not processed immediately by the system go in holding status and are placed in a separate queue (the Apex flex queue).

Up to 100 batch jobs can be in the holding status. When system resources become available, the system picks up jobs from the Apex flex queue and moves them to the batch job queue. The status of these moved jobs changes from Holding to Queued. Queued jobs get executed when the system is ready to process new jobs and they will then go to InProgress status. This would give you an impression that a lot of jobs are in holding status and are stuck. They actually are placed in the Apex flex queue and are waiting to be picked up when system resources are available. "