That's quite a vague question you've got there skipper.
Sunglasses come in many types. Their primary old-school purpose is to reduce the amount of light reaching your retina so as to not cause eye strain etc.... On a camera you can do the same thing by stopping down your f-stop, or adding an ND filter.
The second most popular feature of sunglasses is to protect eyes from ultra-violet light, which people cannot actually see, but can do damage to your eyes nonetheless. It just so happens that film can see UV light, so it's nice to put a UV filter on your camera too. I don't think that's what you're looking for, though.
A fairly common feature people look for in sunglasses is polarization. Filters and sunglasses with this property absorb light waves coming in at a certain pitch, allowing you to cut down on glare.
Finally, sunglasses come in all sorts of different colors, most of which can be easily matched through other filters, or in post etc...
Be more specific with your questions in the future and I'm sure we'll be more helpful.
Nota "That one guy... You know, the one that made that post... About that topic..." Mono