*deeeeeep breath*
There are two main differences - the size of the image sensor and the quality of the lenses. Note that I say size, not pixel count. One of the things that drives me barmy is when people equate pixel count with quality. It's basic physics that if you jam more sensors into the same area, the noise returned increases. Thus, my aged Canon 10D (6Mp) is vastly superior in terms of image smoothness and noise in dark areas to a P&S with 12Mp. It has a larger sensor area and the size of its individual sensor buckets is much larger.
Also, bigger sensor = more area to gather light which (usually) equals less noisy / smoother pix.
Then, DSLRs have detachable lenses. If you're buying a lens that costs three times what an entire P&S camera costs, there's a good chance that money has gone into superior optics. Less fringing, sharper images, less chromatic aberration all add up to "better" images. Old-school shutterbugs see a camera body as something you hang off the back of a nice lens. I've got loads of Canon lenses now - I won't be buying new ones as they're high quality. Any future camera will either have to fit them or I'll buy an adaptor ring. Think of lenses as like your studio monitors: tech may advance in softsynths, DSPs, audio convertors etc. but a great set of speakers will remain a great set of speakers. They're a worthwhile investment.
GENERALISING ENORMOUSLY people seem to like the shallow depth of field that you can get with a DSLR + right lens + right settings. The pictures seem more 3D to us and the photog can choose where s/he concentrates our attention. In most camera phones and p&s, DOF is not so easily controllable or even possible. This gives us flat images where everything is in focus. Great for landscapes or macros, maybe, not so pretty for portraits or creative effects.
Here's
a good linky about DOF.These two main differences add up to why DSLRs are lovely: they offer the photographer greater control of the image-making process. It's kind of like the difference between a home keyboard and a synth.
*phew*
Is that okay as a little intro?
