Extract[expr, list] extracts the part of expr at the position specified by list.
Extract[expr, , , ... ] extracts a list of parts of expr.
Extract[expr, ... , h] extracts parts of expr, wrapping each of them with head h before evaluation.
Extract[expr, i, j, ... ] is equivalent to Part[expr, i, j, ... ].
The position specifications used by Extract have the same form as those returned by Position, and used in functions such as MapAt and ReplacePart.
You can use Extract[expr, ... , Hold] to extract parts without evaluation.
See The Mathematica Book: Section 1.8.4.
See also: Part, Take, PadLeft.