The sustain pedal or damper pedal is often simply called "the pedal", since it is the most frequently used. It is placed as the rightmost pedal in the group. It lifts the dampers from all keys, sustaining all played notes, and altering the overall tone.
The soft pedal or una corda pedal is placed leftmost in the row of pedals. In grand pianos, it shifts the entire action, including the keyboard, to the right, so that the hammers hit only one of the three strings for each note (hence the name una corda, or 'one string'). The effect is to soften the note as well as to change the tone. In uprights, this action is not possible, and so the pedal moves the hammers closer to the strings, allowing the hammers to hit the strings with less force and produce a softer sound.
On grand pianos, the middle pedal is a sostenuto pedal. This pedal keeps raised any damper that was already raised at the moment the pedal is depressed. This makes it possible to sustain some notes (by depressing the sostenuto pedal before notes to be sustained are released) while the player's hands are free to play other notes. This can be useful for musical passages with pedal points and other otherwise tricky or impossible situations.
On many upright pianos, there is a middle pedal called the 'practice' or celeste pedal. This drops a piece of felt between the hammers and strings, greatly muting the sounds.
There are also non-standard variants. On vertical pianos, the middle pedal can be a bass sustain pedal: that is, when it is depressed, the dampers lift off the strings only in the bass section. This pedal would be used only when a pianist needs to sustain a single bass note or chord over many measures, while playing the melody in the treble section. On the largest Fazioli piano, there is a fourth pedal to the left of the principal three. This fourth pedal works in the same way as the soft pedal of an upright piano, moving the hammers closer to the strings