1) System Design Basics: Scalability | Consistency | Availability
Systems design a procedure by which we define the architecture of a system to satisfy given requirements. It is a technique by which the required amounts of scalability, reliability, performance and consistency are satisfied in real world systems.
Horizontal scaling is adding more machines to deal with increasing requirements. These machines handle requests in parallel to improve user experience.
Vertical scaling is replacing the current machines with more advanced machines to improve throughput and hence response time. The techniques are used in conjunction in real-world systems.