A buffer overflow occurs when a program or process tries to store more data in a buffer than it was intended to hold. Since buffers are created to contain a finite amount of data, the extra information — which has to go somewhere — can overflow into adjacent buffers, corrupting or overwriting the valid data held in them. 

In buffer overflow attacks, the extra data may contain codes designed to trigger specific actions.

[1] http://searchsecurity.techtarget.com/definition/buffer-overflow