Resistors

Resistors simply change the amount of current flowing through a specific part of your circuit. We usually need resistors for protecting sensitive components (like e.g. a LED) that would be damaged by a large current.

You can calculate the current flowing through a resistor as

I = V / R   [current = voltage/resistance]

Usually you buy resistors in packs of 600 or so, 10 of a kind packaged together on a banderole. Besides the number on the banderole indicating the resistance each resistor can also be identified through its color stripes, which are used to encode the resistance. Keep the package that usually contains a legend to the color codes or refer to this wikipedia article. Also note that resistors are not directional. However it’s easier to decipher the color codes if you apply them all in the same direction.

For the circuits described on this site (e.g. Project Blinkenlights), resistors with any power rating will do. If you later plan to construct higher-power circuits, you will need to ensure that the power rating of your resistors is at least equal to the desired resistance times the (higher voltage) current squared (P = I² · R).