Although 'Faster' was closest, so far you have not gotten an answer that reflects the traditional naval architectural definitions of tender and stiff. These terms are strictly used in reference to initial stability (form stability) and generally are not thought to include other forms of stability. A boat that is said to be stiff has a lot of form stability. (Stability that comes strictly from the shape of the boat.) Tender refers to a boat without much form stability.
Faster explained quite well the shape of a boat that produces lots of form stability, but in a general sense, as they heel, boats with lots of form stability shift their center of buoyancy to leeward more quickly than tender boats. Since the center of gravity does not shift relative to the boat itself, as a boat with a lot of form stability (a stiff boat) heels, initially, the lever arm between the center of gravity and the center of buoyancy increases rapidly, creating a lot of initial stability. The problem occurs at steeper heel angles, Unless the boat has a very low center of gravity, as a stiff boat heels the center of buoyancy rapidly moves back toward the center of gravity. As this occurs the stability of the stiff boat can rapidly decrease.
Stiff boats tend to have very quick motions but rock through smaller roll angles. They also tend to have a smaller angle of ultimate stability and so are more prone to be able to capsize and stay over longer.
Under the traditional definition of tender or stiff, neither term has anything to do with the overall displacement of the boat, the depth of its keel, or the amount of sail area that it carries. Traditional heavy displacement boats tend to be tender (lacking form stability). Modern race boats tend to be a bit on the stiff side and 70's era race boats tended to be excessively stiff.
What confuses people is the frequent coloquial missuse of the terms. People assume that tender means that a boat heels easily. That really opens a whole can of worms because at that point, a boat that appears stabile may be so for a lot of reasons varying from a low center of gravity (which is good for performance, seaworthiness and motion comfort) to a lot of form stability, (which is not bad for performance but comes at the price of motion comfort and seaworthiness) or simply short of sail area (which does nothing good for the boat in the long run).