[UPDATE] The development of TrueCrypt was stopped in 2014 and is no longer maintained. If you need something similar, you can check this post for alternatives: https://www.comparitech.com/blog/information-security/truecrypt-is-discoutinued-try-these-free-alternatives/
TrueCrypt is free open-source disk encryption software for Windows, Mac, and Linux. You can use it to encrypt your files, disk partitions, and even a whole disk. It is very easy to use and the encryption and the decryption process is transparent to end user, for instance, once a TrueCrypt container (a container is a TrueCrypt encrypted file containing all your data) is decrypted and mounted, the mounted container is the same as a disk drive to the end user. Any file put in the mounted disk drive will be encrypted automatically, and any file pulled outside the disk will be decrypted automatically. As many open-source projects, it relies mainly on donations from users for their development cost. Recently, TrueCrypt set up a donation goal of $150,000 for their future development. My donation is limited, so I would like to use my blog to give it a shout-out. If you are a happy TrueCrypt, I encourage you to consider a donation to TrueCrypt to help them meet their goal. To see the current donation status and make a donation, you can visit their home page at http://www.truecrypt.org Happy Encrypting!