Usually a woman who is willing to have sex with anyone. Sometimes for money, but the real word for that is a Prostitute. A "Ho" can also be called a "Slut" or "Whore". The word most likeley derived from the word "Whore", being the H-O part; w-ho-re.
Usually misspelled Hoe, which is a gardening tool.
guy 1, "Hey man, I just got laid by a ho last night.."
guy 2, "Nice, I just got laid two nights ago with by this slut named Maria and...."
Maria, "What the hell you talkin' 'bout!?!"
guy 2, "Oh Maria! I..."
Maria, "Shut up fool! I ain't yo ho!"