There are lots of really cool hidden gems built-in to OS X that allows you to offer a rich set of flexible features, while maintaining a high level of security and protect your system (and data) from malicious behavior. The Guest account built-in to OS X is one of those features.
Most homes have a family computer located in a public area (kitchen, great room, etc). Most small offices have the same thing. It's a computer generally located in a public place for some employees or household members to use, but at the same time, can be used for visitors or guests to check their email, surf the web, etc. In most cases, the guest user asks an authorized user to log in under their account, but this can cause several problems. Not only is it inconvenient for both parties, but it also poses a security risk to both parties. OS X has a built-in Guest user account. This limited account option is there for one reason... to allow guests to temporarily log in to a computer with no risk or inconvenience to either parties.
To enable the Guest account, open your System Preferences and click on the "Users & Groups" button. As a system administrator, you will need to unlock the preferences by clicking on the lock icon and entering your password. Once this is done, highlight the Guest user account on the left side of the screen, then check the "Allow guests to log in to this computer" box on the right. It will look like this:
With the Guest user account enabled, anyone can log in (without having to use a password) to your Mac. At this point, (I hope) you're asking yourself, "Wait, if anyone can log on to the computer, isn't this a HUGE security risk?". Let me walk you through a couple really cool features of the Guest user account.
First of all, the Home directory of the Guest user account is deleted every time the Guest user logs off. If they created files, documents, set preferences, etc... that's all gone. Even if they change the wallpaper, it reverts back the next time they log in. This keeps that account profile clean and secure (the next user to log in to the Guest account starts with a clean slate).
As for accessing computer settings and other users' data... that doesn't exist. As a Guest user, they aren't allowed to make any changes that will affect other user profiles or the system, and they certainly aren't allowed to access data stored in any other user profiles or Home directories. It's as if they are using a stale, out-of-the-box Mac.
One more note with Guest user accounts... you will notice on the screenshot above that you have the option to enable additional Parental Controls. This will give you access to set time limits and other restrictions associated with Parental Controls.
In short, this is a very nice feature of OS X, one that you shouldn't overlook if you expect a lot of guests coming through your home or office.