In the beginning, things were simple: you had two strings (a username and a password) and if someone knew both of them, they could log in. Easy.
But the ecosystem in which they were used was simple too, for example in MIT's Time-Sharing Computer, considered to be the first computer system to use passwords:
This work is licensed under a Creative Commons Attribution 4.0 International License. In other words, share generously but provide attribution.
Opinions expressed here are my own and may not reflect those of people I work with, my mates, my wife, the kids etc. Unless I'm quoting someone, they're just my own views.
This site runs entirely on Ghost and is made possible thanks to their kind support. Read more about why I chose to use Ghost.