Application Programming Interfaces (APIs)
In his article titled "What's an API?", Justin Gage defines APIs by writing, "Applications are just a bunch of functions that get things done: APIs wrap those functions in easy to use interfaces so you can work with them without being an expert.
Packy McCormick goes on to mention that APIs are what allows software to interact with other software.
An example that I like to use when trying to explain APIs is part of Uber's tech stack.
Uber has many functions in their app such as payments, text/call and GPS but Uber didn't create the code for these functions because it would take a lot of time and work to do this from scratch.
What they do instead is use APIs built by other companies such as Google Maps for GPS, Twilio for text/call and Stripe for payments.
The image on the left shows a bunch of the leading APIs today categorized by their function and with software being such a critical component of our daily lives these days, I view APIs as one of the most interesting opportunities from a tech stack standpoint and from an investment standpoint.