A cleft sentence is a complex sentence with a meaning that could be expressed by a simple sentence. This type of sentence puts a particular constituent into focus. In conversation, this focus is often accompanied by special intonation.
Here are examples of cleft sentences:
It's you I love.
It's Leonard who we're looking for.
That was the prince you were talking to.
What she wanted to buy was a new house.
A new house is what she wanted to buy.
In the cleft sentence a specific element is emphasized by placing it in a separate clause. This element is often introduced by the pronoun it. The cleft sentence divides a single message into two clauses.