Grin coin will be the first real life product of a proposed bitcoin privacy tech- MimbleWimble. You can read the basic of MimbleWimble here: https://steemit.com/bitcoin/@sames/the-future-of-bitcoin-privacy
MimbleWimble is near the bottom but read my whole post and get enlightened! :)
So thanks too reddit user KnifeOfPi2 here is the diff between Grin and Monero:

In short, Grin is about stripping things away - stripping everything away - down to the bare bones of a private blockchain.

Basically, Monero has three privacy protocols.Ring signatures, stealth addresses, and ringCT. However, stealth addresses and CT are the only parts that are absolutely essential to privacy. Ring signatures are great for the paranoid, but they actually aren’t completely necessary. They’re wonderful, but they’re also redundant. Grin strips these away and leaves only stealth addresses and confidential transactions. An observer still can’t derive any useful information from a Grin transaction, and the fact that Grin allows transactions to aggregate, or merge together, makes it even harder to trace.

One of the issues with Monero is blockchain bloat. All this privacy leads to very large transaction sizes. So how do you deal with this? Some solutions like bulletproofs create smaller transactions, but even then you won’t be able to reduce the blockchain size by more than ten times - and it’ll still be bigger than a Bitcoin blockchain of equivalent complexity. This is bad for nodes, because eventually people will run out of space.

So Grin does something pretty darn cool. Let’s say A sends 10 Grin to B, and B later sends that 10 Grin to C. This will be recorded in the final blockchain as A—>C! B gets cut completely, because his ownership of the coins did not change the final state. Now of course nobody will know who A and C are, because of stealth addresses. This is great for cutting down blockchain size because intermediate transactions can be stripped away.

The last thing Grin strips away is something that literally every other cryptocurrency - even Monero - has. With Grin, the entire concept of addresses is eliminated. They just don’t exist. All transactions are created by direct wallet-to-wallet interaction!! Not only is this really cool, but it can be done over email, or any other method. The last great thing about killing addresses is that there is literally nothing to link you to. Even if you reuse the same wallet, there’s no need to create new addresses. Your privacy cannot even be violated by someone with whom you’ve transacted, because you have no address.
Add Tor/VPN protection and privacy is further increased.

Isn't that just cool?!!!
I can't wait to see Grin coin. I heard they might be doing some type of a bitcoin pegging.

And who says bitcoin does not find any new tech.