This is all new to me.

This is beyond what I was taught or have worked with, but the main point about traditional databases were that they always guaranteed consistency, ACID and all that. Very important for most data storage.

However once you start breaking a database into multiple servers distributed across the globe, the costs of maintaining consistency between those individual database servers rises exponentially, until it becomes impossible to have something like Google or Facebook work in anything close to real time.

So you have distributed storage where changes propagate throughout the system but the database servers are never 100% consistent with each other.

I'd be very unhappy if my bank balance varied according to which ATM machine I was using at the moment, but if some of my friends see my status update immediately and others see it a few minutes latter, or if I get slightly different google results ... its not a big deal.