"That was a good movie, wasn't it?"
"You couldn't wash the dishes, could you?"
"When you don't call and then come home late it upsets me, doesn't it?"
The above are examples of tag questions. A tag question is a fragment of a question attached to the end of a declarative clause. Normally, a positive clause gets a negative tag and vice versa. You can have a positive clause with a positive tag ("You can, can you?) but it is not as common and serves a different function (perhaps expressing disbelief) than common tag questions. Invariant tags are not clause dependent and are usually used for simple confirmation, "You booked the tickets, right?"
Common tag questions (like the ones at the top) can serve different purposes in conversations1. I see them mostly as a way of facilitating conversation, and at times, a coercive method of beginning confrontational conversations. If you say "I didn't like that movie, did you?" than the tag is a way of starting a conversation about the movie while also offering your own opinion as a springboard. If you say to your child "Eating cookies when you're not supposed to eat cookies is bad, isn't it?" then you're using the facillitative nature of tag questions to coerce your child into a conversation they would rather not have. Instead of saying "You took the cookies", you present the confrontation like it's question/conversation which, I feel, softens the initial part of the ensuing argument. "Joey you're supposed to be in bed, right?" is an invariant tag which reduces the authorative nature of telling Joey to go to bed.
So, tag questions can be used to soften declarative clauses and it's this softening nature of tag questions which Robin Lakoff's pioneering work on language and gender, "Language and Woman's Place", discussed. Lakoff (who said women used tags most often) argued that the way men and women learned how to speak, the lexical and semantic choices they make, were learned in childhood and mirrored, as well as reinforced, the inequality in american society. Tag questions were supposed to be one example of how women's language was "weak" and showed a lack of assertiveness ("Go to bed Joey" is certainly more assertive then "Your supposed to be in bed, aren't you?"). While Lakoff was super important for beginning this type of analysis of language, the view of tag questions as inherently weak, or used predominantly by women, has largely been dismissed by linguists.
1the meaning and purpose of all language is altered by context and inflection so I'm trying to use common examples where these should be implied. | 
