Let's start with A0 which is in the bottom right corner, and work our way counter-clockwise
A0 (a.k.a D12) - This is a special pin that can do true analog output so it's great for playing audio clips. In can be digital I/O, or analog I/O, but if you do that it will interfere with the built-in speaker. This is the one pin that cannot be used for capacitive touch.
A1 / D6 - This pin can be digital I/O, or analog Input. This pin has PWM output and can be capacitive touch sensor
A2 / D9 - This pin can be digital I/O, or analog Input. This pin has PWM output and can be capacitive touch sensor
A3 / D10 - This pin can be digital I/O, or analog Input. This pin has PWM output and can be capacitive touch sensor
A4 / D3 - This pin can be digital I/O, or analog Input. This pin is also the I2C SCL pin, and can be capacitive touch sensor
A5 / D2 - This pin can be digital I/O, or analog Input. This pin is also the I2C SDA pin, and can be capacitive touch sensor
A6 / D0 - This pin can be digital I/O, or analog Input. This pin has PWM output, Serial Receive, and can be capacitive touch sensor
A7 / D1 - This pin can be digital I/O, or analog Input. This pin has PWM output, Serial Transmit, and can be capacitive touch sensor