A rant is an argument that is fueled by passion, not shaped by facts. When the shouting starts on talk radio, or when a blog commenter resorts to ALL CAPS — you're almost certainly encountering an instance of ranting.
Rant comes from the Dutch ranten, "to talk nonsense." Rave is a close synonym — in fact, "to rant and rave" is a popular expression. When rant is used as a noun, it means something like tirade. The first recorded usage of rant is from the end of the sixteenth century, in Shakespeare's The Merry Wives of Windsor. By the middle of the turbulent seventeenth century, the name Ranters was used as a catchall pejorative for various groups of radical Christian dissenters.