1. The extension cord method. Simply put, loads are unplugged from the wall outlet and attached to a portable generator with extension cords. There is no interfacing between utility and generator power.
2. The circuit transfer switch. Most commonly known as the "GenTrans" switch, these manual switches allow for the re-routing of circuits individually through separate transfer switches (either 6 or 10) within the transfer device. All transfer switches must break the connection from one power source (utility or generator) before making the connection to the other and vice versa. This practice eliminates the possibility of the two power sources coming together. This type of switch is designed primarily for portable generators where individual circuits can be switched to prevent overloading.
3. The load transfer switch. This is a singular switch which performs the same task as 2 above, but with higher capacity to switch the whole house or an emergency sub-panel. Load type transfer switches are available in manual transfer switch (MTS) or automatic transfer switch (ATS) varieties.
   
   Besides starting/stopping the generator and transferring power automatically, an ATS will also perform a generator exercise program (a generator will get "atrophy" much like we do without periodic exercise) and operate the emergency power procedure when unsafe utility conditions (brownouts) exist as well as power failures (blackouts). An ATS requires a stationary generator with an engine controller.

Unless you are very competent at electrical wiring, you should have a licensed electrical contractor perform the electrical installation involved in 2 and 3 above. In most areas, you will need to obtain an electrical permit for electrical installation. If you are connecting to your natural gas source, a plumber will be needed for fuel hookup. If you are connecting to propane, your propane supplier will perform the fuel source installation. Stationary units usually require a startup service performed by the manufacturer's representative after installation to activate the warranty.

-- Tips courtesy of HomeAdvisor.com