Klout is an online service that measures your influence by analyzing your interactions on social networks. If you have a public Twitter account, your Klout profile is automatically created, regardless of whether you signed up for the service or not.
Other Internet services, like search engines, can access your Twitter feed as well, so Klout isn't unique when it comes to gathering your Twitter data. Klout, however, allows you to opt-out of their service. Here's how: