The ATtiny85 is part of the ATtiny family of microcontrollers, originally from Atmel, now Microchip Technology. It is very small, with only eight pins, six of which are available for I/O. Despite its limitations, it's surprisingly accessible to beginners (myself included) thanks to its compatibility with the Arduino IDE.
The clock speed of the ATtiny85 can be set using one of several internal clock sources, or an external one (like a crystal oscillator) for full flexibility. The default setting utilizes the internal 8 MHz oscillator combined with a clock divider of 8, resulting in a 1 MHz clock speed. This clock divider can be disabled to run at the native 8 MHz. Additionally, there's another clock source called the Phase-Locked Loop (PLL), which doubles the internal frequency to achieve a 16 MHz clock speed.
In the world of microcontrollers, a fuse is not only a hardware device that protects circuitry from electrical overload, it's also a concept of low-level configuration settings stored in non-volatile memory. These fuses are typically used to set hardware configuration parameters like clock sources and boot sequences, things that are essential when initializing the microcontroller. Once set, these fuses remain set until explicitly changed, even if new firmware is flashed into the program memory.