Noclick hooking text

Hello everyone!
I encountered a very serious problem.
It is very important to me.
So help me.

In Electronic Dictionary(or Cyber Dic or Computer platform dictionary), if user moves and puts mouse cursor on any text, the Dic program acknowledge the word under mouse cursor and display the meanings of that word.
What I want to know is how the Dic program acknowledge the word under the mouse cursor automatically and realtime.

It seems to use Message Hooking and etc.
Please respond, and upload any sample if you can.
Help me!