DiagonalMatrix

gives a matrix with the elements of list on the leading diagonal, and 0 elsewhere.

gives a matrix with the elements of list on the k diagonal.

pads with 0s to create an n×n matrix.

Details

For positive k, DiagonalMatrix[list,k] puts the elements k positions above the main diagonal. DiagonalMatrix[list,-k] puts the elements k positions below.
DiagonalMatrix[list,k] fills the k diagonal of a square matrix with the elements from list. Different values of k lead to different matrix dimensions.
DiagonalMatrix[list,k,n] always creates an n×n matrix, even if this requires dropping elements of list. »
DiagonalMatrix[list,k,{m,n}] creates an m×n matrix.
DiagonalMatrix[SparseArray[…],…] gives a SparseArray object.