DNS stands for Domain Name System that translates domain names, which are in letters, to IP address which is in computer language (numbers and dots). If there is a problem with your ISP (Internet Service Provider), such as not able to open a website, another alternative is to use OpenDNS service which provides a different IP address than the one provided by your previous DNS. Apparently, OpenDNS is recommended by many because it is more reliable, secure and gives more options.
I've never used this service yet, but next time if I can't open my site again, I will definitely use this as another option.
