An interneuron is a neuron that communicates only to other neurons.
One example of interneurons are inhibitory interneurons in the neocortex which selectively inhibit sections of the thalamus based on synaptic input both from other parts of the neocortex and from the thalamus itself. This is theorized to help focus higher attention on relevant sensory input and help block out irrelevant or boring input, such as the sensation of the backs of your thighs on a chair.
Contrast to sensory neurons or motor neurons, which respectively provide input from and output to the rest of the body.