Resistors in a circuit

Resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current through it in accordance with Ohm's law:

V = IR

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome). They can be made of different kinds of materials. They may be carbon film or even metal strip like tungsten.
When connected in series, their value add up and the resulting value is given by,

Req = R1 + R2 + ..... + Rn

When in parallel they are not additive, instead their resultant value goes down, but powers are additive in that as well. So, In high current application this connection is useful, whereas in high voltage application series connection is required.

In parallel, resultant resistance is given by,



They can be

Carbon composition type
Carbon film type
Metal Film
Wire Wound resistors like rheostats

There are also adjustable resistors like rheostats, potentiometers, strain gauges.

Resistance is measured by color code in electronics.
This color code is as follows.

Color	1st band	2nd band	3rd band (multiplier)	4th band (tolerance)	Temp. Coefficient
Black	0	0	×100
Brown	1	1	×101	±1% (F)	100 ppm
Red	2	2	×102	±2% (G)	50 ppm
Orange	3	3	×103		15 ppm
Yellow	4	4	×104		25 ppm
Green	5	5	×105	±0.5% (D)
Blue	6	6	×106	±0.25% (C)
Violet	7	7	×107	±0.1% (B)
Gray	8	8	×108	±0.05% (A)
White	9	9	×109
Gold			×10-1	±5% (J)
Silver			×10-2	±10% (K)
None				±20% (M)

For example orange, orange and yellow makes 330 kilo-ohms.
334 = 33 × 10,000 ohms = 330 kilo-ohms

Every resistance has power capacity which should not be exceeded in any condition otherwise the resistance may fail.