1a. I went to a bar last night. (You aren't being specific.) 1b. I went to the bar last night. (Your friend knows what bar you are talking about.) - - - - - - - - 2a. I went to a party last night. (You aren't being specific.) 2b. I went to the party last night. (Your friend knows what party you are talking about.) - - - - - - - - 3a. I want to go to a club in NYC. (You aren't being specific.) 3b. I want to go to an eighties club in NYC. (You are being specific about the type of club that you want to go to but not specific about what eighties club you want to go to.) 3a. I want to go to the club we went to in NYC last year. (Your friend knows what party you are talking about.)

A is a general article THE is a definate, specific article http://www.englishpage.com/articles/a-an-vs-the.htm