In Euclidean plane geometry, a quadrilateral is a polygon with four sides (or 'edges') and four vertices or corners. Sometimes, the term quadrangle is used, by analogy with triangle, and sometimes tetragon for consistency with pentagon (5-sided), hexagon (6-sided) and so on. The word quadrilateral is made of the words quad (meaning "four") and lateral (meaning "of sides").

Quadrilaterals are simple (not self-intersecting) or complex (self-intersecting), also called crossed. Simple quadrilaterals are either convex or concave.

The interior angles of a simple quadrilateral add up to 360 degrees of arc.

http://en.wikipedia.org/wiki/Quadrilateral

The above explanation is copied from Wikipedia, the free encyclopedia and is remixed as allowed under the Creative Commons Attribution- ShareAlike 3.0 Unported License.

(Our solved example in mathguru.com also uses the below concept. This is our own explanation, it is not taken from Wikipedia.)

To construct a quadrilateral when 2 adjacent sides( x units, y units) and 3 angles(say 105, 105, 60) are known

•       Draw a line segment MO = x units

•       Make an angle of 105at O

•       With O as the centre and radius y units, we mark an arc on the ray OX

•       Make an angle of 105at R

•       Make an angle of 60at M

•       Mark the point of intersection at R

•       MORE is the required Quadrilateral