What is Resistance? & How to check Resistance color code?

Resistance:

Resistance is the opposition that a substance offers to the flow of electric  current.It is represented by the uppercase letter R.  The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega: 

When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm. (For more discussion of the relationship among current, resistance and voltage, see Ohm law.)

In general, when the applied voltage is held constant, the current in a direct-current (DC) electrical circuit is inversely proportional to the resistance.  If the resistance is doubled, the current is cut in half; if the resistance is halved, the current is doubled.  This rule also holds true for most low-frequency alternating-current (AC) systems, such as household utility circuits. In some AC circuits, especially at high frequencies, the situation is more complex because some components in these systems can store and release energy, as well as dissipating or converting it.

The electrical resistance per unit length, area, or volume of a substance is known as resistivity. Resistivity figures are often specified for copper and aluminum wire, in ohms per kilometer.

Opposition to AC, but not to DC, is a property known as reactance.  In an AC circuit, the resistance and reactance combine vectorially to yield impedance.

Resistance contrasts with conductance, which is a measure of the ease with which electrical current flows through a substance.

The resistance (R) of an object is defined as the ratio of voltage across it (V) to current through it (I), while the conductance (G) is the inverse:

${\displaystyle R={V \over I},\qquad G={I \over V}={\frac {1}{R}}}$

Resistance Color code:

In the sections below examples are given for different numbers of bands, but first some tips are given to read the color code:

• The reading direction might not always be clear. Sometimes the increased space between band 3 and 4 give away the reading direction. Also, the first band is usually the closest to a lead. A gold or silver band (the tolerance) is always the last band.
• It is a good practice to check the manufacturer’s documentation to be sure about the used coding system. Even better is to measure the resistance with a multi-meter. In some cases this might even be the only way to figure out the resistance; for example when the color bands are burnt off.

Some YOUTUBE Video About Resistor:

Resistorguide.com