text: A node is a point along a standing wave where the wave has minimum amplitude.  For instance, in a vibrating guitar string, the ends of the string are nodes.  By changing the position of the end node through frets, the guitarist changes the effective length of the vibrating string and thereby the note played. The opposite of a node is an anti-node, a point where the amplitude of the standing wave is at maximum.  These occur midway between the nodes.
  Explanation
  Standing waves result when two sinusoidal wave trains of the same frequency are moving in opposite directions in the same space and interfere with each other.  They occur when waves are reflected at a boundary, such as sound waves reflected from a wall or electromagnetic waves reflected from the end of a transmission line, and particularly when waves are confined in a resonator at resonance, bouncing back and forth between two boundaries, such as in an organ pipe or guitar string.
  In a standing wave the nodes are a series of locations at equally spaced intervals where the wave amplitude (motion) is zero (see animation above).  At these points the two waves add with opposite phase and cancel each other out.  They occur at intervals of half a wavelength (λ/2).  Midway between each pair of nodes are locations where the amplitude is maximum.  These are called the antinodes.  At these points the two waves add with the same phase and reinforce each other.