If you move your mouse over to the left side of this page to where the green menu pops out and click on "Showcases" you can get some good information. There is an interview with a Showcase Director and an article on Tips for showcases. Its a good place to start.
During a showcase you can expect to face good competition. A position player will be timed on foot speed take batting practice and fielding drills in front of scouts/coaches. Games/scrimmages with live pitching are typically played following the initial drills. A pitcher will typically throw 2-4 innings in those games.
After a showcase, if you perform well and catch someone's eye, you may get a letter or two from a college coach or a questionnaire from a scout. Some showcases will either send you a report or post one on their website.
Showcases are a fixed part of HS baseball players trying to get to the next level. My advice is to not attend one if you're out of playing shape. If you're a pitcher, make sure you're well rested in advance. You'll probably be a little nervous, but just remember that all you're really trying to do is find out if you can play well against the best. Its a chance to get "graded" and see where you fit.
Good luck!