Quora is very different from StackExchange sites. There are no badges or karma. Users aren't ranked. The site revolves around following people, questions and topics that interest you and your homepage reflects those. The design is much cleaner and pages update in real time. You also pull in your social connections and can follow your friends.

One difference is this: StackExchange sites tend to focus on questions that have one objective, correct answer. StackOverflow, ServerFault, the math site, etc., all put the emphasis on having a single canonical resource for people asking questions about whatever topic. (There are exceptions to this, like programming.stackexchange.com, which is basically the "subjective questions" version of StackOverflow.)