OpenDNS is an alternative Domain Name System (DNS) resolution service. DNS is the phone-book of the internet. Every website has a unique IP address. OpenDNS connects the domain name (e.g. google.com) with its IP address. By default, your ISP (internet service provider) will use their own DNS. Your ISP can collect information about your internet surfing habits and sell that information to advertisers (who sometimes will target you with pop-ups). OpenDNS helps to address this issue along with providing some other advanced features.
OpenDNS adds features such as misspelling correction, phishing protection, and content filtering (optionally). They are the fastest publicly available DNS resolution service. This is, in part, due to their cloud network with strategically placed servers.