Perception is the process by which organisms interpret and organize sensation to produce a meaningful experience of the world. Sensation usually refers to the immediate, relatively unprocessed result of stimulation of sensory receptors in the eyes, ears, nose, tongue, or skin. Perception, on the other hand, better describes one's ultimate experience of the world and typically involves further processing of sensory input.
There are many theoretical accouns of perception, which can, by and large, be divided into two groups; bottom-up theories (or data-driven) and top-down theories. Top-down theorists start their explanation of perception from the top, focussing on expectancies, prior knowledge, and other higher-level cognitive processes and then work their way down to considering the sensory data such as perceptual stimulus. Bottom-up theorists start from the bottom and consider the perceived physical stimulus, the observable form or pattern, and work their way up to higher-level cognitive processes such as the organising principles (Sternberg 1996).
James Gibson's theory of "direct perception" (see affordances) is a theory of perception which is often designated as bottom-up (but really ought not to be) and which is well-known in the HCI community. According to Gibson's theory of direct perception, the information in our sensory receptors is enough to perceive anything. We thus perceive "directly" because we do not need any higher-level cognitive processes to mediate between our sensory experience and our perception (Sternberg 1996).
Thus, perception in humans describes the process whereby sensory stimulation is translated into organized experience, whether that be on a top-down or bottom-up basis.