Workflow controls the life of a particular object and any checks that are needed as it passes between different stages.
For example, when you as to join a group you may be subject to checks by the administrator of the group to make sure you are suitable before you become a full member. If you want to leave then the administrator may also check that it is appropriate that you do so.
Content that users contribute has the most sophisticated workflow. There may be checks that what you have written is of the required quality and it could be returned if it is not. It may also be subject to review at some time in the future to ensure that what it says is still current.
When workflow checks require manual intervention a task will appear in the user's My tasks.