Horizontal Scaling vs Vertical Scaling
Horizontal Scaling is also known as Scaling Out. Vertical Scaling is also known as Scaling Up
Horizontal scaling means that you scale by adding more machines into your pool of resources. Example : As web traffic goes up you add more web servers to take on the traffic.
Vertical scaling means that you scale by adding more power (CPU, RAM) to an existing machine.
One generally scale horizontally in Cloud