If you lease an apartment, the person to whom you pay rent is your landlord. A landlord owns a house, building, or property and rents it out to other people.
Many people have landlords they call when their drains are clogged and mail their rent checks to on the first of each month. Business owners often have landlords too, who own their retail or office spaces. Men and women can both be landlords, although you can also call a female landlord a landlady. The concept of a landlord goes back to feudalism — the lord part comes from "Lord of the Manor."