CryptSync is a free open source utility that synchronizes multiple files in between a pair of folders and encrypts the content of one of them with the aim to upload the encrypted data to the cloud keeping the original unencrypted files locally, synchronization works both ways, whenever there is a change in one of the folders it replicates into the other, the utility also encrypts file names as they sometimes reveal details, the files are all separately encrypted and have the extension .cryptsync. You could also store data inside an encrypted Truecrypt container and upload it to the cloud but you will have to update everything manually while CryptSync automates the process, the idea is to use this program to store encrypted data online with minimum effort, and it does a good job at that.
Encryption is implemented with 7-Zip, an open source archiving software that highly compresses files, saving space, if you need to open an individual encrypted file in the cloud you can save it to your hard drive and open it with 7-Zip together with your CryptSync password. Software features are minimal, a “Start with Windows” option, “Run in the background” and “Create a New Pair“, you have to be careful when you erase a folder pair because no confirmation is asked for, but no data will be lost even if you erase the pair by mistake, only the settings are erased, you can use this application from the command line too.
There is no help manual included but the author has a very complete explanation on how CryptSync works on his website. I would not use this tool if you already have an account with a specialist privacy focused cloud company like SpiderOak or Teamdrive since their software already encrypts your data locally before reaching their servers and they have no access to the encryption keys or backdoor. CrypSync will be useful in shady cloud storage services that have minimum security or built-in backdoors, like for example DropBox, where the company employees can access the encrypted servers where your data is stored, you could also use this utility in a network, securely storing backup files inside a NAS (Network Attached Storage) and keeping the original ones inside your fully encrypted computer.