A resistor has a series of three to four color bands on it.  Each band represents a number.

Black = 0
Brown  = 1
Red = 2
Orange  = 3
Yellow = 4
Green  = 5
Blue = 6
Violet = 7
Grey = 8
White  = 9

The first two bands represent the most significant digits in the value of the resistor. The third band is called the multiplier. It indicates by what power of 10 the most significant digits are multiplied by. The fourth band is the tolerance band.  Gold being 5%, silver being 10%.  Resistors having only 3 bands have a tolerance of 20% or more.

OK so how do we translate these bands into a value?

Example 1
Lets take a resistor that has Brown, Black, Red, and Gold bands.  The first two are the most significant digits.
Brown = 1, Black = 0.  So we have 10.  On the multiplier band we have a Red.  Red = 2.  This tells us to multiply 10 x 10 or 10^2 which = 100.  Now we multiply our first two bands with the third. 10 x 100 = 1000 ohms or 1K ohms.  The Gold band tells us that the tolerance is within 5%.  Meaning the value could be as low as 950 ohms or as high as 1050 ohms.  This resistor has a value of 1K ohms with a tolerance of 5%.

Example 2
Lets take a resistor that has Yellow, Violet, Orange, and Silver bands.  The first two are the most significant digits.
Yellow = 4, Violet = 7.  So we have 47.  On the multiplier band we have a Orange.  Orange = 3.  This tells us to multiply 10 x 10 x 10 or 10^3 which = 1000.  Now we multiply our first two bands with the third. 47 x 1000 = 47000 ohms or 47K ohms.  The Silver band tells us that the tolerance is within 10%.  Meaning the value could be as low as 42300 ohms or as high as 51700 ohms.  This resistor has a value of 47K ohms with a tolerance of 10%.