FileVault is a built in application on your Mac that allows you to fully encrypt your hard disk.
To check the status of file vault within Terminal type the following:
fdesetup status
To enable FileVault type the following:
sudo fdesetup enable
* Terminal will then ask you to reboot to enable the change. You can repeat this for all user accounts you want to encrypt.
If you want to disable FileVault you can. Type the following into Terminal:
sudo fdesetup disable
If you want more information on the Terminal command you can type the following into Terminal for the help page.
man fdesetup