Why question?
Questioning has a number of important purposes in the classroom.
In the main, its purpose is to check students' knowledge/ understanding.
a) If you've just delivered a lot of information, you want to check that they know what they're doing or you want to discover what they've gained from what you've delivered.
b) It can be a better alternative to you giving all of the answers. If you ask students to think for themselves then the process that they go through, is far more likely to stay with them than if you tell them all the answers.
Top Rules
One top rule to question effectively is to ask specific people to answer. If you throw a question out into the entire classroom, it is highly likely to fall unanswered. The other result you could get is the students who are confident that they know the answer put their hand up. The result of this is that you'll usually get the same people answering and you won't get to check that everyone understands- which is key.
For the most part, 'Why?' and 'How?' will you get you very far in extending questions. If you don't extend a question then students will always think that a mediocre, unsupported answer will be enough.
Acknowledge an answer, but don't state whether it was correct or not. This way, other students might be able to add to one another's answers.