Lord is a title given to men of power and lands in the Westlands. Normally, a lord is a noble member of a house, and the greater power that the house itself has, the greater the power the lord may wield.
In some cases, men who are not of noble blood are still addressed as "lord." This is usually because the person addressing the man feels such great respect and awe toward him that formalities seem necessary.
The female equivalent to a lord is a lady.