DLL injection is a technique used to run code within the address space of another process by forcing it to load a dynamic-link library. DLL injection is often used by third-party developers to influence the behavior of a program in a way its authors did not anticipate or intend. For example, the injected code could trap system function calls or read the contents of password textboxes, which cannot be done the usual way.
In this video, Prasanna K (prasanna.in [] gmail [] com) runs us through the basics of DLL Injection. This is a highly recommended watch!