Most electronics run on direct current (DC) power. This is the type of power that batteries put out, and it is straightforward to deal with when engineering electronics. However, the power that runs through power lines and homes is alternating current (AC) power. Alternating in this case refers to the fact that AC power switches direction back and forth quickly, flowing in both directions along an electrical line. This bidirectional flow would damage DC equipment, which is only capable of handling power flowing in one direction, thus the need for an AC/DC adapter. take for “hp Pavilion DV6000 AC Adapter” example:

Hp Pavilion DV6000 AC Adapter 90W 19V 4.74A Bullet shape tip Description:

http://www.udtek.com/hp-pavilion-dv6000-ac-adapter-90w-19v-474a-bullet-shape-tip-p-4822.html

Input: AC100-240V (worldwide use)
Output: DC19V 4.74A
Power: 90W
Connecter size (Bullet shape tip):
Internal Diameter: 1.6mm
External Diameter: (4.75+4.2)mm
Item Includes: AC Adapter and Power Cord.

Physical Conversion

To convert alternating current into direct current, an AC/DC adapter makes use of an electronic component called a rectifier. This component takes an electrical input and only allows power to flow in one direction out of it, effectively creating DC power. Rectifiers are only capable of handling a certain level of electrical current before they are irreparably damaged, so make sure to check your power adapters before bringing them to location with different levels of AC power. For example, do not attempt to use adapters rated only for U.S. (110 volt) electricity in Europe, where electricity is typically 220 volts. Failure to take this into account could result in damage to your power adapter or electronic devices.
Voltage and Amperage

Unlike AC power, which is well standardized for voltage and amperage within a country, DC power required for a piece of electronic equipment can vary wildly. Make sure not to use a AC/DC adapter on your electronics that is not rated for a different voltage, as you run the risk of damaging your equipment. Using a power adapter rated for a higher amperage than your electronic equipment is safe, as the equipment will only draw the necessary current.