Description:

RealCrypt is a software system for establishing and maintaining an
on-the-fly-encrypted volume (data storage device). On-the-fly encryption
means that data are automatically encrypted or decrypted right before
they are loaded or saved, without any user intervention. No data stored
on an encrypted volume can be read (decrypted) without using the correct
password or correct encryption key. Until decrypted, a RealCrypt volume
appears to be nothing more than a series of random numbers. Entire file
system is encrypted (i.e., file names, folder names, contents of every
file, and free space). RealCrypt never writes decrypted data to any
storage device (it only temporarily writes data being decrypted to RAM).

Based on TrueCrypt, freely available at http://www.truecrypt.org/.

RealCrypt is mainly just a rebrand to allow for modifications to take place,
functionality remains all the same.