Text messages, MMS, email that is stored locally or cached, notes, voice memos, browser cache and history, etc. It does not include the OS. The usage bar in iTunes only shows useable space. The total amount is a few GB less than the capacity of the phone because the OS isn't counted there.
A normal other section only has a few GB. It starts out less than 1 GB on new devices, but climbs with usage and storage. Never clearing out old texts is one of the biggest culprits of a large other section. You can see a breakdown of how much space each app is taking in Settings > General > Usage. If the other section is very large and there is nothing to explain that on your usage page, it is likely corrupt media files. The only way to clear it is to restore.
