Here following we will discuss about the resistors classification according to application and colours code.

Classification according to the application
Resistors can be classified, according to the used application, in many categories. Here following we will describe briefly some of them.
Automotive:
Resistors are largely used in the automotive area. They are used in the: Body Control Systems; Drive Train & Starter/Alternators; DC/DC Converters; Lighting Supplies; Engine Management & Ignition/Injection Systems; Electrical Replacement of Mechanical Functions; Hybrid/Electric Vehicles. Metal film series, by Welwyn Components for example, are suitable to all the above mentioned automotive areas. They have a power rating, at 70°C, from 0.4 to 0.75 Watt, a limiting element voltage from 200 to 350 Volts, tolerances down to 0.5% , a temperature coefficient down to 50 ppm/°C. To learn more @:
http://www.welwyn-tt.com/pdf/datasheet/MFR.PDF

MFR series - Courtesy of Welwyn Components

Aerospace:
Last September Vishay Intertechnology has launched in the market the new VPR221Z ultra-high-precision Z-Foil resistor. This resistor, in TO-220 configuration, has a TCR ( Temperature Coefficient of Resistance ) of ± 0.05 ppm/°C, a PCR ( Power Coefficient of Resistance ) of 4 ppm/W and Load Life Stability of ± 0.005 % (50 ppm). To learn more @: http://www.vishay.com/docs/63116/vpr221z.pdf

VPR221Z - Courtesy of Vishay Int.

Medical:
In the medical area the resistors are used in the : Defibrillator Charge Control; Defibrillator Pulse Protection; ECG High Gain Amplification; X-Ray Supply; Ultrasound & MRI (Magnetic Resonance Imaging) Scanners. Precision Measurement VSMP series, by Vishay Int., is based on Bulk Metal Foil Technology. These resistors are excellent for applications that require high precision and stability. They have TCR: ± 0.2 ppm/°C typical, PCR : 5 ppm at rated power, Load Life Stability (70°C for 2000 hours): ± 0.005%. To learn more @: http://www.farnell.com/datasheets/71876.pdf

VSMP series top view

High Voltage resistors: Nicrom Electronic, by its thick film technology, can reach tolerances until 0.05 %, temperature coefficients better than 10 ppm/°C, voltage coefficients lower than 0.003 ppm/V and stability less than 0.1-0.2% after 1000 hours at rated power. Its resistors, in the new serie 1000, can arrive up to 400 kV in air. To learn more @ : http://www.nicrom-electronic.com/Serie_1000.pdf

Precision resistors:
HR/HVA/HVS ultra precision resistors, made by PRC, have values from 0.1Ω to 10 MΩ , tolerances from ± 0.01 % to 0.005 %, stability until ± 0.001 % year at 25°C, temperature from -65°C to 125°C. To learn more @ :
http://pdf.directindustry.com/pdf/precision-resistor/prc-2008-product-ca...

Precision resistors - Courtesy of PRC

Surge Resistors:
STEC-028 surge resistor, Supertech Electronic manufacturer, can suffer until a 10kV discharge current. To learn more @ : http://supertech.manufacturer.globalsources.com/si/6008800039268/pdtl/Me...

STEC - 028 / Courtesy of Supertech Elect.

Telecom resistors:
RK73 is a series of resistors that targets high performance telecommunication applications. RK73-RT (flat chip resistors ) is the last update release. These resistors have an optimum anti-sulfuration characteristic, very good heat and weather resistance, high stability and reliability due to the triple-layer structure of electrode. To learn more @ : http://www.koaspeer.com/pdfs/res68.pdf

RK73-RT Series / Courtesy of KOA

Avionics:
The electronics equipments installed in the planes and dedicated to their driving are indicated by the "avionics" term. The TMC series thick film chip resistors meets both avionics and military high reliability requirements, including aerospace systems, weapon systems, aircraft and communication equipment. This series has a failure rate of 0.01% or even less. To learn more @: http://news.thomasnet.com/companystory/516311

TMC thick film chip resistor - Courtesy of IRC

Classification according to the colours code
Resistors can be classified, according to their resistive value, by a sequence of coloured rings that can be 4 for the standard ones, 5 ÷ 6 for the precision ones.

Standard resistors:
In these resistors the first and second ring are the resistive value, the third is the multiplier, the fourth is the tolerance.

Precision resistors:
In these resistors the first, second and third ring are the resistive value, the fourth is the multiplier, the fifth is the tolerance and the sixth is the temperature coefficient.

SMD resistors:
They have printed a number with three digits: the first and second digit indicate the resistive value, the third indicates the numbers of zero to be added. For example: 211 = 2-1-0 = 210 Ω .
At every colour is associated a number, as you can see in the below table:

To read the resistive value correctly it is better to put the tolerance values (generally gold) on the right. Let's finish this subject reading the resistive value of the below resistor:

the first ring (starting on the left) is brown =1; the second ring is black = 0; the third ring is black = 0; the fourth ring (the multiplier) is brown = 10; the fifth ring (the tolerance) is brown = ± 1%. So the resistive value is 1-0-0 x 10 = 1000 = 1k Ω with tolerance of ± 1 %.

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Read also:
Base Electronics: Resistors Configuration
A Guide to Resistors