What determines what shows up in the bash history command?

When I run history, I get a list of commands. However, it does not appear to be complete, even for a given period of time. (It doesn't feel like it has continuous coverage of the past N commands - it feels like some are missing.) Also, the output of history is different in different terminal tabs. Is this because the history isn't written to disk until the tab is closed? Does that explain why certain commands seem to get lost, because they are overwritten by closing tabs?

I'm on OSX 10.8 for what it's worth.

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