A passive linear circuit element that presents an electrical resistance between two nodes in a circuit. Resistance is measured in Ohms ( 
).
Their value is represented by a colour coding scheme. Manufacturers offer a range of resistor values.
There are different parameters to consider when selecting a resistor, including:
tolerance % – a measure of deviation from the intended value
power (W) – the maximum sustained power that can be dissipated by a resistor
form factor – there are various sizes and shapes, ranging from ultra small surface mounted devices to components with large metal windings.
materials – different materials exist, each with different properties including ability to dissipate heat, tolerance, thermal stability and noise.
Identification and colour codes – There are some great online sites for working out the value of resistors.
We recommend the page from Digikey
http://www.digikey.co.uk/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band