The exterior derivative of a differential form of degree k is a differential form of degree k+1. Exterior differentiation satisfies three important properties:
It can be shown that exterior derivative is uniquely determined by these properties and its agreement with the differential on 0-forms (functions).
Special cases of exterior differentiation correspond to familiar differential operators of vector calculus along the same lines as the differential corresponds to the gradient. For example, in 3 dimensional Euclidean space, exterior derivative of a 1-form corresponds to curl and exterior derivative of a 2-form corresponds to divergence.
This correspondence reveals about a dozen formulas from vector calculus as merely special cases of the above three rules of exterior differentiation. The kernel of d consists of the closed forms, and the image of the exact forms (cf. exact differentials).