“Who’s” is the contracted form of who is. It is pronounced the same way as “whose” (/ huwz/), but the meaning is different.

 Didn’t you meet her …? Notice that this is a negative question. Charles thought that Jim had met Mary before. He is now surprised that Jim does not know Mary, and so he uses a negative question to show his surprise.

 I wasn’t at Steve’s party. Notice that the emphasis here is on “at” although prepositions normally have weak stress. In this case, “at” means “there” (I wasn’t there).

 Mary, this is my friend Jim.  This is a friendly way to introduce two people. It’s common to follow this with “Jim, this is Mary.” In this case, Mary says “Hi, Jim” first.

 Nice to meet you. This is a typical response after you’ve been introduced to someone.

 “Sure” is often used in informal conversation to mean “yes.”