Scaling comes in a few different flavors. Vertical scaling involves growing the computing power of a single server, adding memory, faster or more CPUs and/or faster disk I/O.
Horizontal scaling involves adding additional computing resources or servers in parallel and then load balacing across them.
Scalability refers to applications which facilitate scaling well. With web applications, the middle tier aka the webservers are fairly easy to scale horizontally and most enterprise class applications already do this with commercial load balancers – with either hardware or software.
Doing the same with the database tier, however can be trickier. Enter MySQL replication to facilitate a fairly painless horizontal scalability. Build your application architecture with read-only transactions, and write/update transactions segmented apart, and you can send the latter to one master database, and the former to a handful of replicated slaves. With a typical web application that is less than 10% writes, and 90% reads, there is the potential to add as many as 5-10 servers horizontally to increase application throughput by as much as 500-1000%.
Sean Hull asks on Quora: What is scalability and why is it important?