confusion about the 5V and 3.3V on the arduino uno board.

Im extremely new to arduino and microcontrollers in general. I have learnt about digital, analog ports and PWM signals but im struggling with the point of the 5v and 3.3v ports. For example when i put an LED into pin 13 and GND and ran the blink program it worked fined and so I was wondering the purpose of the 5v and 3.3v when electricity runs through the normal pins. Can someone explain its purpose in its simplest terms. Also i just wanted to clarify if GND or ground is for completing the circuit

I was wondering the purpose of the 5v and 3.3v when electricity runs through the normal pins.

While electric current flows from the pins you can only let 40mA or less flow without damaging the pins.

The 5V line allows you to supply more current and switch it with a small current from the pins by using a transistor.

The 5V line also allows you to sink current into a pin as well as what you think is normal ( but it is not ) sourcing current from a pin.

The 3V3 line is limited to about 50mA and does not get used much but can power low current low voltage accessories, providing they draw less than 50mA.

Thanks for the answer. I kind of understand it now but could you use it in a circuit example?

This is about driving motors and relays:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html

Yes the GND pin is used for the return path to the +5 and 3.3volt pins.