The dominant architectural style today is the horizontally scaled farm of commodity hardware. Horizontal scaling means we add capacity by adding more servers. We sometimes call these “farms.” The alternative, vertical scaling, means building bigger and bigger servers—adding core, memory, and storage to hosts. Vertical scaling has its place, but most of our interactive workload goes to horizontally scaled farms.
If your system scales horizontally, then you will have load-balanced farms or clusters where each server runs the same applications. The multiplicity of machines provides you with fault tolerance through redundancy. A single machine or process can completely bonk while the remainder continues serving transactions.