Overexposure is the term used to describe an exposure that is brighter than what would be considered a "neutral" exposure. Creatively speaking, overexposure could be a tool used intentionally for a special effect, however when most photographers refer to an image as being "overexposed", they are implying that the exposure is either brighter than what ought to be "right" for the scene, or that the exposure is actually so bright that some (or a lot) of the image has become pure white. In digital photography, major overexposure is generally more difficult to correct in post-production, compared to underexposure. Highlights that are too overexposed may not be recoverable at all, and will look unnaturally white or grey when too much "highlight preservation" is used in post-production.
Overexposure: Creative and Accidental
Technically speaking, an overexposed image is simply a very bright image. When photographers talk about "correct" exposures, or overexposure or underexposure, however, they may not always be referring to the same thing.
In some cases, "correct" to a photographer means perfect neutral grey. (That's 18% grey, to most people.) However, as many will point out, perfectly "neutral" is one of the first of the rules meant to be broken in photography, and in many cases, a perfectly neutral exposure is simply not the "right" exposure for a scene.
If overexposure is what makes a photograph look right, then overexposure would be considered correct. This is of course a subjective matter, however in many cases, such as a snowy landscape, a brighter exposure would actually appear more natural and realistic than a perfectly neutrally exposed image.
See also, High Key Photography.
With this in mind, in most cases when a photographer speaks of overexposure, they are almost always referring to an error, whether technical or creative. If an image is too overexposed, its highlights (and sometimes almost everything!) will become so bright that there is no image recorded at all, just pure white. These are known as clipped or blown out highlights. Recovery of blown highlights in a raw image is possible up to a certain point, however any clipping of highlights is risky; eventually the recovery of highlights will lead to unnatural posterization. (abrupt tonal transitions that should appear smooth)