User Activity

want to know the difference of use Dev, Prod , Test instances as a developer?
what is the purpose of sys_history_line
can  someone please tell me , in how many ways we can create record producers? need simple answer to understand
what is scratch pad,  what is its use in business rules , client scripts, ui policies , workflows? what is difference between scratchpad and ajax in clientscripts ?
can someone please explain these things? retroactive start? retroactive pause ? when to cancel? set start to?