Monopods are lighter and far more portable than tripods. But they're also not as versatile? unless you add a little extra support.
Monopods, otherwise known as one-legged tripods, have been popular with sports and outdoor photographers for years. They give you that extra bit of stability to help keep your photos as sharp as a tack.
The problem with monopods is that they can't stand on their own. Often this isn't a problem, unless you want to capture really long exposures of 1/4 of a second or more.
Those are the times that you yearn for your tripod. The fact of the matter is that many photographers don't like lugging tripods around. This is especially true on long hikes, when you're trying to keep your equipment weight to a minimum. Happily, there's a solid compromise that is stable, versatile, and, most importantly, not too heavy.