Master Key is the password you got when you first signed up for steemit. and you can keep that.

Posting key is used for posting and voting. This must be different from the active key and the owner.

The active key is used to transfer and order in the internal market.
The active key that you can get by clicking login to appear with your master Key account steem,

These are the basic difference of all keys and these keys helps us to work on dmania.lol but these keys also secure us so that only specific permissions are granted to our account.