Tr[list] finds the trace of the matrix or tensor list.
Tr[list, f] finds a generalized trace, combining terms with f instead of Plus.
Tr[list, f, n] goes down to level n in list.
Tr[list] sums the diagonal elements list[[i, i, ... ]].
Tr works for rectangular as well as square matrices and tensors.
See The Mathematica Book: Section 3.7.7.
See also: Transpose, Det, DiagonalMatrix, Eigenvalues.