Arduino Uno R3 Pinout
Arduino Uno R3 headers, board oriented with the USB socket to the left. Check your board against its silkscreen before wiring power.
Two headers, one board
The left column is the power and analog side, the right column the digital side, in silkscreen order. The SCL and SDA pins beside AREF are an R3 addition and duplicate A4 and A5, so shields stopped stealing analog pins.
Current limits
About 20 mA per pin is comfortable (40 mA absolute maximum) and 200 mA total across the chip. An LED is fine, a motor is not, which is what transistors and drivers are for.
Serial pins
D0 and D1 are shared with the USB-to-serial converter. Anything wired there fights the uploader, so keep them clear while you are still flashing sketches.
Sources and datasheets
This diagram was cross-checked against the manufacturer documentation below plus at least two independent references before publishing. When they disagreed, the manufacturer won.
More boards and plugs live on the pinouts index.