Helmholtz Theorem
A vector field is uniquely determined by its divergence and curl, given the boundary conditions.
A vector field without curl is called irrotational and a vector field without divergence is called solenoidal.
If \(E\) is irrotational, \(E = - \nabla V\) for some \(V\).
If \(B\) is an solenoidal field then \(B = \nabla \times A\) for some \(A\).
In general any vector field is sum of an irrotational and solenoidal field.