There are three cases depending on the sign of the expression under the square root:
i) b2 < 4mk (this will be underdamping, b is small relative to m and k). ii) b2 > 4mk (this will be overdamping, b is large relative to m and k). iii) b2 = 4mk (this will be critical damping, b is just between over and underdamping.