By its very definition, a quadrilateral is merely a shape with four sides and four vertices or corners. The prefix "quad-" simply means "four" and lateral means "sides," so the name explicitly lays out its meaning.
The most common expression of this is found in squares, but other quadrilaterals include the parallelogram, rhombus, rectangle and trapezoid. The only real restrictions are that all sides must be straight, the shape itself must be closed, and the vertices must meet at an angle. Otherwise, the lines can be of any length and the angles used can be of any degree necessary to satisfy these characteristics.