scale out VS. scale up.What is Database Scale-Out?

scale out (by increasing the number of servers)
scaling up (by adding more memory and CPUs to the servers)
What is Database Scale-Out?

Scale-Out is a modern computing architecture that enables organizations to improve application performance and scalability on an incremental, as-needed basis by adding multiple replicated database servers on low-cost commodity hardware. This is in contrast to a Scale-Up approach, which requires organizations to make a large up-front investment in more expensive and complex server hardware and database licenses in order to add capacity.
In the online world, many of the largest and fastest-growing companies use MySQL to cost-effectively Scale-Out their successful businesses, saving millions of dollars over high-cost proprietary technology — including Google, Yahoo, craigslist, Ticketmaster, Wikipedia, YouTube, and Evite/Citysearch.