Mitre joints connect the ends of components at different angles.
An example would be the corners of picture frames, where the corner is a right angle, and the mitre joint is made at forty five degrees (assuming the connecting sides are both the same width).
Mitre joints are inherently weak, since the mating surfaces will be end grain, and so many variations have been developed to add strength to the 'simple mitre'.
Tenon and Mitre
Butterfly Keyed Mitre
Veneer Keyed Mitre