On most electrical panels there is a main breaker. The main breaker does as the name implies, it is a main disconnect for the entire panel board. Today, this breaker is typically 200 amps or more and is normally located at the top of the electrical panel or on the bottom.
A home electrical panel is made of two bus bars located behind the circuit breakers. Each of these bus bars are 110 volts and are a separate phase. If you were to look at the power on an oscilloscope, you would see the two phases are exactly 180 degrees out of phase. This is how you make 220 volts, by using the two phases.
Often when a main breaker fails, only one phase is effected by the failure. Thus it is not uncommon to have power to one half of the house. When one finds power to have of the house a simple way to check if it is a lost phase is to turn on the electric stove, if a phase has been "lost" the other half of the 110 volt outlets in the home will begin to work. The reason is the power is passing through the elements on the stove and to the un-powered phase in the electrical panel. When this occurs there can only be two possible causes, either the power company lost a phase coming into the panel or the main breaker has gone bad.