The ATTiny85 is an Atmel AVR board that is a small in form. It is like the little brother to that ATmega32p that is found in the Arduino boards. It has all the same kind of functions but with fewer GPIO pins.

• Official product page: https://www.microchip.com/wwwproducts/en/ATtiny85

SparkFun has a nice pinout diagram: https://learn.sparkfun.com/tutorials/tiny-avr-programmer-hookup-guide/attiny85-use-hints

SparkFun also has a nice one-page PDF reference guide for ATtiny85: https://cdn.sparkfun.com/assets/2/8/b/a/a/Tiny_QuickRef_v2_2.pdf

The `o` represents the circle indentation in the board.
Align the `o` to the top-left where pin 1 is.

 Reset  1 -o==- 8  5V
 A3/D3  2 -   - 7  D2/A1 (SCK)
 A2/D4  3 -   - 6  D1 (PWM, MISO)
 GND    4 -===- 5  D0 (PWM, MOSI, AREF)

Use an Arduino as ISP to program the ATtiny85.