an Implementation is what we call a peice of software that follows some specification. There can be many implementations and they may not use the same code to implement the specification. An example of this is Web Browsers the all implement HTML handling code which in theory reads html in the same way as there are html standards.
Object Oriented Programming[]
In Object Oriented languages Interfaces are said to be implemented. This allows programs to treat several different classes in the same way as long as they implement the same interface.