TrueCrypt is a free open source on-the-fly encryption (OTFE) program for Microsoft Windows XP/2000/2003 and Linux. It allows one to create a virtual encrypted disk (TrueCrypt volume) within a file and then mount it as a real disk. TrueCrypt can also encrypt an entire hard disk partition or a storage device/medium, such as floppy disk or USB memory stick. Thus, TrueCrypt creates device-hosted TrueCrypt volumes. Everything stored on a TrueCrypt volume is entirely encrypted (i.e., including file names and folder names). TrueCrypt volumes behave as real physical disk drives. This means that it is possible, for example, to repair the encrypted filesystem with chkdsk, defragment mounted volumes created by TrueCrypt, etc.
The encryption algorithms supported by TrueCrypt include AES, Blowfish, CAST5, Serpent, Triple DES, and Twofish. It also allows the use of a cascade of different ciphers, for instance AES+Twofish+Serpent.