gpedit.msc vs rsop.msc

What is the difference?

Gpedit is for editing the policy; RSOP, as its name implies, gives you the Resultant Set Of Policy/ies. It's more of a troubleshooting tool, to find out just what policies are being applied to the particular user/computer, and from which GPOs.

Like if I am forced to use complex passwords, though I changed it in the default domain policy, I can run a RSOP on my computer and see which GPO is still enforcing the restriction. It will show me all the policies currently applied to my account/computer, from all the affecting GPOs.