The metric tensor is a second rank (specifically, it is a (2,0) tensor) tensor $g_{\mu\nu}$ such that $g_{\mu\nu}=\hat e_\mu\cdot\hat e_\nu$. It therefore describes the angles between vectors. Curvature tensors can be derived from it.

This tensor is commonly used in general-relativity, where the curvature of spacetime describes the strength of gravity, in a sense. A solution to the Einstein Field Equation is a solution for the metric tensor.

The metric tensor formulation is of course, a second-order formulation. The first-order formulation uses the vielbin.