What are the main differences between a regular Digital camera and a DSLR?
Well, what do you mean when you say "regular digital camera"?
I understand you're talking about a point and shoot, but a DSLR is also digital. It's basically just the way it operates.
SLR = Single Lens Reflex
per Wikipedia - a type of camera that uses a movable mirror placed between the lens and the film to project the image seen through the lens to a matte focusing screen.
There's film and there's digital. The main difference is how the image is recorded.
An SLR, digital or film has a mirror that projects the image viewed through the lens into the view finder. When the button used to take a picture is snapped, the mirror lifts and the shutter opens, either exposing film or a sensor that records the image based on lens settings and camera settings/film speed.
The difference between an SLR and a digital point and shoot is that the sensor on the P&S is exposed all the time. Digital SLR's generally have much better sensors than in the P&S cameras, allowing for better pictures. an SLR also has the ability to change lenses mounted to the front of a camera, giving you a very good way of getting any picture you want (although at a price, some lenses run in the tens of thousands. Not many, but some) and are generally more flexible with more options for a photographer.
They allow for every aspect of the camera and lens to be set including aperture, shutter speed and ISO. Most P&S cameras do not let you do this. There are some out there that do, but then again, you're stuck with one lens.
SLR's offer amazing flexibility. If you're interested in getting into photography and learning how to take amazing pictures, you should look into them. If you're just needing a camera to tote around and take a few snaps, a P&S is a good way to accomplish this without breaking the bank.