Steem = blockchain.
Steemit = website skin to interact with Steem blockchain.
MSPSteem = website skin like Steemit.
The different? Steemit gives 100% reward to you. MSPSteem give 95% reward to you, 5% to minnowsupport.
But, if you post via MSPSteem, you have double chance of winning this Weekly Resteem Contest.