Not too much. There is primary and extended for the most part. Some people think of logical drives as some. It's all basically have multiple spaces that you can allocate with something else whether it means splitting the hdd up or having many hdd in there.
Always backup your data if you haven't or never tried partitioning before. If you do it wrong, it could result in you losing your data. Know which hdd you are partitioning in case you might mix them up if you have two or more. Also, familiarize yourself with the program and know the differences between creating a partition, deleting it, and resizing it as they all obviously do different things.
Sure, should work fine. However, most programs still put data in the system32 folder so either way, some info is still being written onto the primary drive.