Névé refers to a layer of snow that has been partially melted and then refrozen. It is granular, hardened snow. If you are skiing this is all you really need to know about the technical aspect of névé. Apparently névé makes for good skiing, but as I have never skied and never will, this is not something I can confirm personally.
In geology, glaciology, and related fields, the definition of névé is a matter of some confusion. It is often used interchangeably with firn. Firn comes from German, meaning 'of last year', and refers to 'old snow'. Névé comes from French, and means 'a mass of frozen snow'. As you can see, there is a lot of potential for overlap. Névé traditionally refers primarily to snow at the accumulation area of a glacier, snow that is not yet ice, but is consolidated and granular. Eventually this snow will be compacted into ice, and eventually become part of the glacier.
Although névé is dense, settled snow, and may have layers of ice embedded within it, it is still permeable. While névé is generally spoken of as being created by a melting and refreezing process, snow packs composed of compacted snow (compressed by the weight of newer snowfalls above it) can also be referred to as névé. This sort of snow pack is often found in Arctic climates.
The terms névé, firn, and old snow can sometimes be considered synonyms. If an author uses both névé and firn, but doesn't define them, he is probably using firn to mean the topmost, newest layer of névé.
vuo says re névé: It's good for skiing because it has a high solid content and compresses just in the right way. Fresh snow is soft and gives in, but is slippery under the ski.
Névé should not to be confused with the Portuguese and Italian words 'neve', which just means snow (n.). The added accents have a radical effect on pronunciation; névé is pronounced 'Nay-Vay'