How is Domain Authority (DA) calculated?

After looking in the Digotal Point marketplace, mainly the websites and domain marketplaces, i have noticed alot of member listing there domains and mentioning the DA / Domain Authority of them. Up until now, I have really thought much about this and I also don't know the DA of my websites / domains. How is it calculated? and also what things help better it and also make it less? One more thing.. Is there a website I can check the Domain Authority of mine?

See original post by Sean101