Relationship between voltage, current, and resistance

V=IxR 
Voltage = Current(amps) x resistance  

1 ohm allows one amp of current to flow when you have 1 volt 

if you move the equation around you can calculate for any of the three unknowns 

I(amps)=V/R

R=V/I(amps) 

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resistors are used to limit the amount of *current* that flows in a circuit 

if the voltage in a circuit remains the same, the current will decrease if you increase the resistance (add resistors)

many components will have a larger apetite for current than they can handle
LEDs have low resistance, and will burn themselves out easily without resistance

@ if you know what the supply voltage is and you know how much current you need, you can use ohms law to determine the right resistor to use for the circuit

for LEDs
a few milliamps of current is enough to make an LED glow, a few hundred milliamps is enough to destroy it

@ resistor can go either on the positive or negative end of the LED - just wired in series (within the same direct path)

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@ calculate the right resistor with ohms law and such

voltage is known 

you need to *decide* how much current you want to give the component, based on its datasheet 
(aim below the max rating)

knowing these two you can calculate the resistor value

you can generally round *up* if the exact number doesn't fit 

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@VOLTAGE DROP = FORWARD VOLTAGE (for data sheets and such)
@Forward voltage is how much voltage it takes to activate the LED
and therefore is subtracted in the equation because that much is used up in the circuit already

- Take supply voltage, subtract the voltage drop of component 
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