It is not easy to answer this without knowing the entire sentence in which these words are used. So Let me try to give to examples.

1- Davide evde yok.

2- Davide evde yoktur.

In the first sentence, the person saying this sentence is 100 % sure that Davide is not at home. Example: A housemate of Davide is answering a phone call. He/She knows that Davide is not at home otherwise she would have seen/noticed him in the house .  

In the second sentence, the person  is not 100 % sure but the probability is very high. 

Example: The person knows that Davide goes to church on Sunday mornings. And on a sunday morning when someone ask that person to call Davide to jogging. He can say "Davide evde yoktur."

Sorry. I agree with first comment. In the first sentence ( yok ) you are sure, and second sentence you are not sure (yoktur ), just prediction

There is no difference actually. 

Yok- no

Yoktur- there is no :)