Delete[expr, n] deletes the element at position n in expr. If n is negative, the position is counted from the end.
Delete[expr, i, j, ... ] deletes the part at position i, j, ... .
Delete[expr, , , ... , , , ... , ... ] deletes parts at several positions.
Example: Delete[a, b, c, d, 3] .
Delete[a, b, c, d, 1, 3] .
Deleting the head of a particular element in an expression is equivalent to applying FlattenAt to the expression at that point.
Example: Delete[a, b, c, 2, 0] .
Deleting the head of a whole expression makes the head be Sequence.
Example: Delete[a, b, 0] .
See The Mathematica Book: Section 1.8.6.
See also: Insert, MapAt, ReplacePart, FlattenAt, DeleteCases, Drop, StringDrop.