When there is increasing amount of load on a system/device we can increase its performance either by adding extra hardware or by upgrading the current system configuration this is known as scalability. 

the two types of scaling are vertical scaling and horizontal scaling.

Horizontal scaling

 In horizontal scaling we had more instances to the existing devices/servers.
In horizontal scaling the performance of the group of servers is increased by adding more instances of the server to your existing group of servers so that the load is decreased .

individual server capacity is not increased, but the load on the server is decreased. 

Horizontal scaling is also known as scaling out.

Horizontal Scaling

Vertical Scaling

Vertical Scaling the capacity of a single device/instance is increased so that the load is decreased on that device. processing power, storage, memory, faster CPU Processors, RAM etc. are added to an existing device.

Vertical Scaling is also called the Scale-up approach.

Vertical Scaling