Habituation is a quite interesting concept. Basically, our brains' are designed to tune into change in the environment. Thus, we tend to tune out, or not pay special attention to predictable and unchanging stimuli. This is habituation. For example, the first time you sense something, your brain will automatically pay more close attention to it, then every other time you sense that something. Eventually, we will grow bored with the stimuli, and could possibly tune it out completely.
I find this particularly interesting because you can apply this to your relationships in your life. For example, comments from strangers are always more exciting or moving then from long time friends and lovers. A mistake many make is leaving a perfectly good relationship, thinking they are bored, but will just habituate to a new person soon.
Keep in mind that this is almost the opposite of sensory adaptation. When sensory adaptation occurs, the sensory receptors grow dull and stop sending as many messages to the brain, whereas habituation is dealt with strictly in the brain.