What are AC and DC?
AC, or Alternating Current, is a type of electricity generated by rapidly changing the poles of a generator (alternator) from one polarity to the other. This causes the the voltage and current to Alternate from positive to negative and back again. AC frequencies are measured in Hertz (Hz) and multiples of Hz.. (i.e. KiloHertz (KHz), MegaHertz (MHz), GigaHertz (GHz) ) The Hertz.. formerly known as the cycle, has been renamed for Heinrich Hertz.
DC, or Direct Current, is a type of electricity which, unlike AC, does not alternate, but remains at one polarity, i.e., positive is positive, and stays that way. It can be generated mechanically, as with a generator, chemically, as with a batter, or by reaction of certain materials with light, such as photovoltaic cells (solar cells). All electronic components require DC to operate.
How is AC converted into DC?
AC is converted to DC by RECTIFICATION. Rectification take the AC wave and basically chops the top (or bottom) of the wave off, leaving a Pulsating DC, or a DC that changes in voltage from zero to the maximum of approximately what the original AC voltage was. The pulsating DC is then filtered using a series of capacitors and resistors or coils to make a constant DC voltage.  This DC voltage is generally 1.414 times higher than the original AC voltage, due to AC being measured at it's RMS value (root-mean-square) rather than it's PEAK value. The resulting DC comes from the AC voltage's PEAK value, which is 1.414 times the RMS value.
Resistors
Resistors are components that restrict the flow of electricity. The unit of resistance is the OHM.  There are various types of resistors available:
Composition- made mostly of carbon
Metal film- made of thin spirals of impure metals
Wirewound- made of wrapped nichrome wire (used mostly for high power resistors).

The two main varieties of resistor are fixed resistors and variable resistors (potentiometer and rheostats).

Most low power fixed resistors have a universal color coding:


                            
First band       Second band     Third band
Black                            0                     0               No multiplier
Brown                           1                     1                   times 1
Red                               2                     2                   times 2
Orange                          3                     3                   times 3
Yellow                           4                     4                   times 4
Green                            5                     5                   times 5
Blue                              6                     6                   times 6
Violet                            7                     7
Gray                             8                     8  
White                            9                     9

In addition, there is often a fourth color band, indicating the tolerance of the resistor value. In this case Silver indicates a 10% tolerance, Gold indicates a 5% tolerance, and Red indicates a 2% tolerance.  When no fourth band is present, the tolerance is taken to be 20%.