Are the 3.3V and 5V pins on an Arduino input or output?
To start off, there are many variants of the Arduino board. Depending on what components are on the board will determine how much current you can draw from the 3.3V and 5V pins. So you're better off checking the schematic of the Arduino you had in mind.

But to answer your question, the 3.3V and 5V pins can be used to power external devices. But I can't say how much current you can pull since that depends on the board design. You can probably power an LED or something, but not charge a phone. The VIN pin is where you supply power to the Arduino. You use this pin if you are powering the Arduino from a voltage source higher than 5V (or 3.3V if that's what the Arduino takes) since the voltage is stepped down to 5V and regulated.