Originally Posted by Praxis

Not exactly, and no. The rear derailleur helps keep the chain on the rear, and the front derailleur helps keep the chain on the front. Under some circumstances, if you don't have a front derailleur or a chain-keeper sort of device (there are a few different designs), the chain can fall off the front. This usually happens more often for chains that are a bit loose and/or for rougher riding while pedaling. From what I've read it can be a bit of a mystery, though--just like chasing down a noise from your bike, it can be hard to figure out why the chain is getting thrown (or why it's not a problem).

Chainrings (that's the front ones) have little ramps and pins on them if they are relatively modern (20 years?) and designed to be used with a front derailleur. These features help the front shifting. They also tend to make it slightly easier for the chain to come off, and there are inexpensive chainrings without ramps or pins.

But a single ramped/pinned chainring can work adequately in the front (and conversely a non-ramped/pinned chainring can work for multiple front chainrings, too).

A chainguard typically doesn't help too much with keeping the chain on. Some of the chain retention mechanisms (designed to keep the chain on, not keep you clean; an example would be a bash ring) will keep the chain off you a bit more.