From: https://www.digikey.pl/pl/product-highlight/a/atmel/attiny85-risc-based-microcontroller

ATtiny85 AVR-based Microcontroller

Microchip offers their high-performance, 8-bit, AVR, RISC-based microcontroller

Operating from 2.7 to 5.5 V, the Microchip AVR tiny85 offers six general purpose I/Os, a 4-channel 10-bit A/D converter, two 8-bit timers, interrupts (internal and external), and the device achieves 20 MIPS at 20 MHz by executing powerful instructions in a single clock cycle. With the programmable memory of 8 KB Flash, 512 B SRAM, and 512 B EEPROM, along with three software-selectable power-saving modes, internal oscillator, and debugWIRE for on-chip debugging, this 8-pin device is a great selection as a space-saving MCU.

With development boards available from Microchip, Adafruit, Sparkfun, or Olimex, there are options and support to get the users project up and running.

A very good resource http://www.interactiondesign.se/wiki/attiny

Another very good resource (now not working 29.05.2020): https://www.theengineeringprojects.com/2018/09/introduction-to-attiny85.html

And 3rd best text: https://www.researchgate.net/publication/327395939_Introduction_to_ATtiny85

Programming Attiny85 using Arduino UNO and Arduino software

Programming an ATtiny w/ Arduino 1.6 (or 1.0)

With USB interface