A Klout Score is the measurement of your overall online influence. The scores range from 0 to 100 with higher scores representing a wider and stronger sphere of influence.
Klout uses over 25 variables to measure True Reach, Amplification Probability, and Network Score. The size of the sphere is calculated by measuring True Reach (engaged followers and friends vs. spam bots, dead accounts, etc.) Amplification Probability is the likelihood that messages will generate retweets or spark a conversation. If the user's engaged followers are highly influential, they'll have a high Network Score.
In January 2010 Trendrr and Klout agreed to share social data. This will integrate Klout's influencer data into Trendrr's intelligence platform. Klout's data tracks Twitter users and their content, interactions with content, and the size/composition of their networks.