A DSLR is a Digital Single-lens reflex camera. Meaning that what you see through the viewfinder is basically what you will see on the picture you take. Wikipedia will give you lots more information on this than you probably care to know.
I'm taking the plunge to a DSLR myself and am really excited for it. Just ordered a Nikon D7100 yesterday and can hardly wait for it to get here. I decided on a Nikon after reading reviews on dpreview.com. I highly recommend checking out their reviews. It's hard to find a more complete investigation and evaluation of a given camera anywhere.