The top operating systems for U.S. smart phones are Google's Android, Apple's iOS, RIM's BlackBerry OS and Microsoft's Windows Phone 7. They are similar in that each allows you to install different applications to extend the phone's capabilities.
The most current version of Google's Android OS is 2.3, sometimes called Gingerbread. It's a slight refinement of 2.2, or Froyo. You may also find phones running version 2.1. However, since both the carriers and handset makers must sign off on Android upgrades, they can take a while to appear - or never come at all. I wouldn't buy a phone with an older version of Android with the presumption that you'll get the latest version later.