The Altair 8800
The Altair 8800 is considered to be the computer that kick-started the personal computer revolution. You can learn more about the Altair on the Altair 8800 Wikipedia page.
Altair 8800 image attribution - Smithsonian Museum
The Altair 8800 was built on the Intel 8080 CPU, the second 8-bit microprocessor manufactured by Intel in 1974. By today's standards, it's a simple CPU design, perfect for learning computing fundamentals because of its small instruction set.
The original Altair 8800 was programmed directly via the front panel. It was a painstaking, error-prone process to load and run a program. The Altair 8800 had a series of LEDs and switches that you used to load apps and determine the state of the Altair.
You could save and load applications from a paper tape reader connected to the Altair 8800. As the Altair 8800 grew in popularity, more options became available. You could attach a keyboard, a computer monitor, and finally disk drives, a more reliable way to save and load applications.