TrueCrypt is an open source program that allows to encrypt files, folders, devices or entire hard drives or partitions. Once you encrypt desired files, only you will have access to that section with a password. It is safe because if anyone get access to your PC or your computer is stolen, they will not be able to access those files.
Even you encrypt files, you will be able to access them with password and use normally because virtual volumes created by this application have a normal behavior, like any other hard drives from PC.
Can encrypt external hard drive or USB flash drive too.
New created volumes or areas where your data is encrypted can be visible or hidden.
Can select different encryption and hash algorithms, the new allocated hard disk space for these files will be formatted before moving files in them. TrueCrypt uses free space to create new volume.