Casper is the thing that makes ETH different and let it be a smart contract Running blockchain?

Or

W/o casper And switch to PoS ETH = NXT ?

Smart contracts are already there & ETH != NXT with or without Casper... think of Casper as Ethereum's POS implementation.

Read the following:

https://blog.ethereum.org/2015/08/01/introducing-casper-friendly-ghost/
https://blog.ethereum.org/2015/12/28/understanding-serenity-part-2-casper/

Sorry, a bit more precise: Is casper the base for the Smart contract executor?