Different boards operate internally at different voltages. This is dictated by the main chip on the board. That main internal operating voltage is also the voltage at which the IO pins operate.
Most "old" Arduino boards operate at 5V. This includes, but is not limited to:
- Arduino UNO
- Arduino Mini
- Arduino Leonardo
Newer more powerful boards, many made by other companies, operate at 3.3V. Again, this includes such boards as:
- Arduino DUE
- Arduino MKR1000
- Teensy boards (v3.0+)
- chipKIT boards
- ESP8266 & ESP32 boards
Some boards come in variants that give either voltage - the most notable being the Pro Mini boards from Sparkfun. These come in both 5V and 3.3V variants and it is important to pick the right board for your needs.
In general, though:
- If it's 8-bit and runs on an ATMega chip it's likely 5V unless otherwise stated
- All 32-bit boards will be 3.3V unless otherwise stated.