It's a tag question, innit?

Tag questions are a kind of yes-no question used when you want to confirm that something is true. They are formed by attaching a question tag to the end of a declarative sentence. Here is one example:

He is in jail, is he?

Notice that the tag "is he" has the same form as the beginning of a normal yes-no question that you would use if you didn't have any idea about the answer:

Is he in jail?

The rules for forming tag-questions are the same for forming yes-no questions (swapping the subject and first auxiliary verb), except you would always use a pronoun in a tag question rather than repeat the whole subject:

Will the man who stole my horse go to jail?
The man who stole my horse will go to jail, will he?

The type of tag question I have just described is for confirming new information:

1. He is in jail, is he? Did I understand you correctly? [confirming new information]

Another kind is for confirming old or expected information and involves negating the auxiliary verb:

2. I just want to check one thing. He is in jail, isn't he? [confirming an expectation]

You would use the first type of tag question when you want to confirm something that has just been mentioned to make sure you understood. The second type is used when you want to confirm information that you believe to be true, but which was not the last thing the other person said.

A third kind of tag question can be used to confirm unexpected information and involves negating the auxiliary in the declarative part of the sentence instead of in the question tag. This is the form that you would use if you were very surprised at the thought of something:

3. He isn't in jail, is he?! I had no idea! [confirming unexpected information]

You can also use this third form to confirm a negative expectation much as you would use the second form to confirm a positive expectation, but in this case you would not use the intonation associated with surprise:

I just want to check one thing. He isn't in jail, is he? [confirming an expectation]

The key to tag questions that are used to confirm expectations is that their polarity must be the opposite of the sentence they are attached to. When the sentence is positive, the tag must be negative and when the sentence is negative, the tag must be positive.

There are also a number of other ways to form tag questions that are less complicated, but achieve the same results. Some are only found in regional variants of English and are stigmatized as very informal or uneducated usage (i.e., as slang):

He's in jail, no?
He's in jail, yeah? [UK]
He's in jail, ey? [New Zealand]
He's in jail, innit? [London]