.COM is the legacy domain extension with more than 100 million registrations. Stick with .com if you're OK with the status-quo. .CO on the other hand is fresh, shorter, social, and... it's available! With an increasing number of people web browsing on mobile devices, the need for short and memorable web addresses has never been so important. In essence, if you want something innovative and cutting edge, go with .CO.
Domain extensions like .info, .mobi, .org and .tv enjoy specific niche markets, which is great if that is what you need for your online branding. .CO is different in that it is more general in meaning and not niche specific.
.CO is registered just like all the other domain extensions. You can get it from domain retailers (a/k/a registrars) for one year or multiple years in advance. As long as you renew your .CO web address, it's yours to keep.