Great questions! Simplifying the message is a very hot topic over here. Like Bitcoin, BitShares represents a range of concepts, rather than just a flagship currency. BitShares is a brand, a protocol, a platform AND a 'currency'. I put currency in quotes because we don't like the currency metaphor around here, we prefer to see BitShares as shares in a decentralized company. This company has products - market pegged assets (bitUSD, bitGLD, etc.), provides a service (decentralized asset exchange), pays employees - the delegates (which are like miners), has voting shareholders (us BTSX holders), AND customers (speculators and savers). Its by coincidence (ok, thoughtful design), that shares in BitShares can be exchanged as frictionlessly as any other existing currency, which is why people are tempted to call it a currency. The real 'currency' aspect of BitShares are its products - market pegged virtual assets.
I'm at work so I can't get into the inner workings of the creation of market pegged assets (or BitShares Music and NOTES), so I'll try to sum it up in the most simple way I can. BitAssets are shorted into existence when a speculator pledges their own shares to the network as collateral. Behind every bitAsset created exists a speculator willing offer bitAsset stability to a 'saver' in exchange for increased risk/reward in trading the volatility of the BTSX share price. You would buy your coffee with bitUSD.