CGI stands for "Common Gateway Interface." It's a standard that allows external programs or scripts to connect with a web server to generate dynamic content and respond to web requests.

In the context of web development, CGI works as follows: When a web server receives a request for a dynamic resource (such as a PHP, Perl, or Python file), it invokes the corresponding CGI script. This script is called with the necessary parameters of the request and then performs a specific task, such as generating HTML, querying a database, or executing computations. The result is then returned to the web server, which forwards it to the client.

CGI was one of the earliest mechanisms that enabled the integration of dynamic content on web pages and laid the groundwork for many later technologies like PHP, ASP, JSP, and others. While it is still used today, faster and more efficient methods such as FastCGI and mod_php (for Apache) or WSGI (for Python) are widely adopted. These technologies offer improved performance and scalability compared to plain CGI.

Horizontal scalability refers to a system's capability to handle increased workloads by adding more resources or hardware components, enhancing its performance. In contrast to vertical scalability, where performance improvement occurs by adding resources to a single node or machine, horizontal scalability scales by adding additional instances of resources that work together.

Typically, horizontal scalability means the system can distribute loads across multiple machines or servers. Cloud computing platforms are often designed to offer horizontal scalability, allowing resources to be dynamically added or removed as needed to enhance performance and availability.

An example of horizontal scalability is expanding a web server by adding more servers to better handle user requests, rather than just increasing the resources of the existing server.

Vertical scalability refers to a system's ability to handle increasing or decreasing workloads by adjusting its resources. In the context of computer technologies, vertical scalability generally means enhancing the performance of a system by adding or removing resources within the same hardware.

In contrast to horizontal scalability, where capacity is increased by adding more machines or nodes, vertical scalability involves improving the capability of a single device, such as a server or a database, by adding more resources like CPU, RAM, or disk space.

Vertical scalability provides a relatively straightforward way to enhance a system's performance. However, there's a limit to how much a single device can scale, constrained by its physical limitations. In some cases, scaling might hit the boundaries of the hardware, leading to bottlenecks. This is why many companies also opt for horizontal scalability to make their systems more robust and resilient.