It encrypts your files and when it's done encrypting, you get a pop up demanding you to pay up $300 in USD, GBP or Bitcoins (and anything else equal to that amount), at this point you have two choices - pay up to get the decryption key to get your files back or start all over/restore from a backup, you don't have any other choices.
This is why having a backup is very important, but even that won't be enough, you'll want to have a backup of the backup stored safely and offline, because it also encrypts the drives/network shares connected to the computer.