Power Supplies
A computer power supply unit (PSU) is the component which AC electrical power into DC power of various voltages used inside the PC. The PSU converts 100-120V American AC power or 220-240 European AC power into low voltage DC power. Some power supplies automatically sense and convert between European and American voltage, some have manual switches, so do not work with different power sources.
Figure 12: PSU with covering off, retrieved from http://en.wikipedia.org/wiki/Computer_power_supply
Power supplies are rated by their maximum power generally from 200 watt to 500 watt. Workstations and high end gaming PCs have power supplies near the upper end of the range, while small form factor and inexpensive PCs are down towards the lower end.
Installing a Power Supply
A power supply should not be disassembled the components on the inside produce too great a chance of electrocution it should always be replaced. Do not remove the case from the power supply change out the entire power supply.
Follow these directions to replace a power supply: http://www.fonerbooks.com/r_power.htm
Display Devices
There are two types of display devices you should be familiar with: notebook screens and external monitors.
Notebook screens are typically LCD and are measured by the diagonal length of the screen. Screen sizes range from several inches to 17 widescreen on most commonplace laptops. Screen resolutions vary, but that is much less of a concern now than it has been in the past.
External Monitors
CRT Monitors
For years, Cathode Ray Tube (CRT) monitors were standard and very few people could afford an LCD screen like a laptop uses. Over the past several years, LCD has become the predominant type of external monitor its hard to even find a CRT monitor (though I have a couple in the basement you can have!).
CRT monitors are known for their size and bulk, but have great picture and color quality. It took several years for LCD to catch up and exceed CRT monitors. CRT monitors range in size from old 12 to 24 and larger.
LCD Monitors
Over the last several years, LCD monitors have become very commonplace and are loved by consumers for their small size and low weight. At one time LCD monitors were very pricey as compared to CRT monitors, but the price gap has narrowed so much as to not be a significant factor any longer.
You must be more careful with an LCD monitor as it is easier to damage than a CRT monitor.