A pedal point (or pedal note) is a note which keeps sounding for several bars, usually in the bass (the lowest notes). They are often used towards the end of a piece of music to help to drive the music to a climax. They are called "pedal" notes because they are often heard in organ music where the player puts one of his feet on a pedal note and holds it there.
A good example of a dominant pedal can be seen in the Prelude in C major from Book I of the Welltempered Clavier by Bach.
An example of a tonic pedal can be seen in the third movement of Brahms's German Requiem, where the double basses play continuous D's in the last section. Because this lasts for the whole section some people might prefer to call this a drone.
A double pedal is two pedal tones played at the same time.
An inverted pedal is a pedal that is not in the bass. It is often in the highest part.