How is the sign (+ or -) of power of a lens related to its divergent or convergent action?

Depending on the direction in which a lens deviates the light rays, its power is either positive or negative.

If a lens deviates a ray towards its centre (converges), the power is positive and if it deviates the ray away from its centre (diverges), the power is negative.

Hence, the power of a convex lens is positive and of a concave lens is negative.