A standing wave is a pattern produced when both reflection and interference occur between identical sound waves (those of the same frequency). The effect is an amplification of the sound made. But this only happens under specific conditions: the right speed and frequency combination must exist of all waves involved.
Standing waves happen when successive waves are emitted from a source and reflect off a fixed boundary. The oncoming waves continually produced by the source collide with the reflected waves to produce instances of interference.
In looking at what a standing wave looks like, it can be said that it is like a series of lined-up lemons. The thicker part of the lemons is where constructive interference happens; where the greatest vertical distance is, is called an antinode. Destructive interference happens between the waves at points called nodes- the tips of the lemons on either side. Nodes are places at equilibrium that do not move at all (they carry no energy). From one node to another is a distance of half a wavelength because they are separated by one lemon (two lemons have the horizontal distance of one whole wavelength). From a node to an antinode is a fourth of a wavelength because it is half a lemon.
Some wonderful animations of standing waves can be found at this site along with a more in-depth description of the topic.