One difference is this: StackExchange sites tend to focus on questions that have one objective, correct answer. StackOverflow, ServerFault, the math site, etc.,

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.)