Dunkin Donuts is a chain of shops which sell doughnuts (I am sure Wikipedia can be enlightening about doughnuts if need be) and coffee. The "dunkin'" part refers to the habit of submerging doughnuts in the coffee to soak some up. This works better with some varieties of doughnuts than others (jelly doughnuts are not generally considered dunkable, for example) and is not a mandatory step in the consumption of either the coffee or the doughnuts.
Canadians refer to such establishments as "Tim Horton's". I have no idea whether Canadian doughnuts are dunkable or not. They are, however, infiltrating the US, and ads may be seen on the sidelines at every NHL hockey game. We live in fear; first their doughnuts, then their single-payer healthcare system. </p>
In the southern US, Dunkin Donuts are impersonated by a chain called Krispy Kreme. People have strong opinions about this, one way or another.