a dslr is a digital single lens reflex camera. in layman's term it allows u to see what your lens is seeing unlike compact cameras through the viewfinder. 2ndly, u can change lenses. but since companies have different lens mounts for their cameras n lenses, not everything is compatible.

in nikon's case, almost all the lenses produced in the last 20 years can be mounted on any dslr. i hope this helps you but buying a dslr without even knowing the basics is a not advisable. dslrs aren't that easy to use and the cost.

since u're getting a d80.. i'll do a sum up for u. the body is around 3600 plus another 2800 for your 18-200. then there's other lenses,macro,wide angle,fish eyes,etc etc. there's tripods too with at the least would set u back a few hundred. external flash?sb-800 isn't that cheap either.i've not gone in filters,storage cards,camera bags, etc.. u willing to invest so much money in something u don't know?

http://en.wikipedia.org/wiki/DSLR extra info for u here. there's abundant info on the net. please do look up