In and out is great way to describe an electrical panel. In every setting, these panels become the hub of an electrical network. Huge amounts of power are brought into the panel to be distributed in lesser amounts to the various components of the network. Some panels drive a single item that needs a lot of power and a dedicated breaker or disconnect. Many outside units for air conditioning equipment require a miniature panel near the unit so the power can be disconnected at the site for maintenance.
In a home, from 1 to 4 panels are usually employed to service the needs of the residence depending on size and house configuration. A very large home can differ little from a mid-sized business in power needs. Regardless of the number of the panels, they all function alike with the exception that some applications have a main panel that can disable all power to the building without pulling the meter.
Generally, power comes into the top of the circuit box or panel. In a residence this would usually be a 220-240 volt input. It will have three large cables to carry between 100 and 300 amps of service to your house. Most houses today are provided with 200 amp service. The top of the box will have one large breaker for 100 amp service with only 120 volts being available, or it will have two large breakers for 200+ amp service with two strips offering 120 volts on each side. Down the outside of the panel will be two long rows of screws that allow for attaching the ground wires in the panel.
Two types of breakers will fit into these boxes for the outgoing service. One is a single width breaker about 1/2-3/4 inch wide. These have clips that attach to only one of the power strips and provide 120 volt service at the amperage listed on the breaker. Most homes employ 15-30 amp circuits. Proper gage wiring needs to be used based on the amp load being carried.
Double width breakers are designed to attach to both sides of the panel at the same time. It will pull 120 volts from each side to produce a 220 volt circuit to run a clothes dryer, kitchen stove, furnace, or other major electrical appliance. Usually, the wiring needs to be much heavier here, and the amp load will range from 30 to 100.
Never work inside an electrical panel unless you have a little training, and the mains have been pulled, or the electric has been disconnected from the building. Failing to do this might result in death. Don't work inside a panel if you have doubts about what you are doing. Call in an certified electrician around Pasadena, CA to handle it.