A Klout score measures an individual's social media influence. Using an app from the company Klout, users can connect various social accounts such as Facebook, Flickr, YouTube, etc., and then provides every user with his or her Klout score. The score is based on a numerical value of 1 out of 100--the higher the score, the more influence an individual has on social media platforms.
Competitors to Klout include PeerIndex, Cred, HubSpot's Marketing Grader and and Crowdbooster, though this list is hardly exhaustive. Whereas a Klout score may be subject to daily changes in social influence, other tools for social influence scoring may focus on one's reach over time, such as PeerIndex.
Each tool has potential, but usefulness may also depend on the nature of the influence to be measured. If a user is active on only one social network, platform-specific tools may work well. If, like most people, you are present on multiple networks, experts counsel to use Klout or a best-of-breed alternative.