Some writers say that writing is like music practice: if you miss a day, you'll notice your work isn't as smooth; if you miss a couple of days, you'll really notice; and if you miss a week, other people will notice. So what can you do when you aren't actively working on a story, novel, poem or essay? You can practice. You can exercise.
A writing exercise is usually something short that you can complete in an hour or two (or even much less), but which gets your mind working and the words flowing. Often if you don't know where to begin on a new project, or just can't seem to get going on a project already in progress, doing a few exercises will bring on the inspiration you need. Most working writers agree that inspiration almost always comes while they're already writing and not before they start.
Targeted Skill-Building
You know those exercises that weight lifters do to target specific muscles or muscle groups? Well, writers can do something like that, too. If you have trouble thinking up things to write about, try an idea-generating exercise. Are your settings little more than backdrops for the action to occur in front of? Maybe a setting exercise would help you find ways to make the most of your settings.
Working on one specific aspect of writing separately allows you to focus on only that aspect in a way you can't really do while trying to write a novel (for example). When you're working on something like a novel or short story, you really need to ignore each component in order to create the whole, or else you'll get so worried about the details you'll never get anything done. Doing exercises for each of those details helps them become a part of your writing faster, so that you won't need to consciously think about them.
Or Just Have Some Fun
Exercising can be fun, too (really, it can). Writing exercises give you an opportunity to play around with words, to experiment in all sorts of ways without