A kite is a quadrilateral with two distinct pairs of equal adjacent sides. 
As a result of this definition:
- Diagonals are perpendicular
- Opposite angles enclosed by two unequal adjacent sides are equal
- Area = half the product of the diagonals
A kite in which all four sides are equal is called a rhombus.
- ↑ There seems to be some debate about whether or not this is a bona fide sub-classification of quadrilaterals, and if so, how it should be defined. See http://www.mathopenref.com/kite.html