A Wiki is fundamentally a collection of information, but its distinguishing characteristic is that it can be edited by (usually) anyone. This means that the body of knowledge within the collection is able to grow very quickly, and can also be updated or added to on a regular basis. The largest out there, and for all intents and purposes the wiki that you should get up to speed on, is called Wikipedia.
This living, breathing archive of information is everything that Encyclopedia Brittanica wishes it was – and much more. In the old stodgy tomes, for example, it wasn’t possible to look up Hip-Hop Slang, or Tool in quite the same way – if at all.
If you value information in any way, shape, or form, make it a point to get familiar with both the concept of a wiki and Wikipedia.