A Square Enix account is a free service provided for authentification in all of Square Enix's online services.
Besides its use in all future online services, one can also associate multiple IDs from current services to a single Square Enix account and enjoy features only available to Square Enix account holders.

A security token is a device that produces a constantly changing one-time password that can be displayed onscreen with the push of a button. Because the device is completely detached from any hardware dealing in user authentication, it provides vastly improved security to the user.

A one-time password differs from a typical ID or password in that it is a 6-digit numeric code that is produced and displayed on the security token's screen each time the token's button is pushed.
One-time passwords are very effective against spyware such as key loggers in that they are only valid for a set period of time and become invalid after they have been used once.