Power comes directly from the electrical company's power line, through the electrical meter on the outside of your house and into the electrical service panel. The "Main" circuit breaker is often called a "Double Pole Service Disconnect" and is what controls the live power from energizing the circuits in the electrical panel. It's easily recognizable as it has two thick black wires feeding it from the electrical meter and consists of two circuit breaker handles joined together. The Main is used to power all the circuits on or off at one time.
The Main circuit breaker also identifies the amperage capacity of the electrical panel and will have a number on it identifying its amp capacity such as "100" or "150". 100 amp service is the minimum allowed by code today, with 150 amp being very common. Electrical panels are also available in 200 amp and 400 amp sizes.