Asset Classes

It occurred to me, for reasons I'll mention shortly, that we haven't specifically discussed the concept of an 'asset class'. There is a universe of assets out there: you can buy stocks, bonds or real estate for example. Some assets trade on exchanges with daily liquidity and pricing while some assets, like direct investments in real estate, have little liquidity and vague estimates of the current value. There are currencies and gold, commodities and collectibles (e.g. art, wine) amongst others.