TL;DR alert.
The power that comes into your house is 240v alternating current in three wires. Two black "hot" wires and a white neutral. The voltage between each of the hot wires and the neutral is 120v. The voltage between the two hot wires is 240v.
Each black wire powers half of the circuit breakers in the electrical panel - half of the 120v breakers are powered by one black wire, the rest by the other black wire. Notice how 240 breaker for your water heater is twice as wide as the 120v breakers? That's because 240v breakers are "double pole" meaning that they draw power from both of the incoming black wires.
You *can* simply install another 240v breaker and wire it to a receptacle into which you plug your generator. It is also hugely dangerous. The power on the poles is some very large voltage, probably 480v. The transformer reduces the voltage at the pole to 240v for use in your house. When the power goes out because a power line is down, anyone with a generator connected in this way is supplying 240v to the transformer, and therefore 480v to the power lines on which the lineman is working.
The solution is called a "lockout" breaker. Depending on how your electrical panel is laid out, it may be possible to install a 240v lockout breaker next to your main breaker that prevents both from being on at the same time. This enables you to power anything in your house without risk of energizing the wires outside your house.
Wiring your house is a lot of hassle and expense and maintaining yet another gasoline engine is too. Something else to consider is your purpose. Do you live in a place in which the power regularly goes out for a day or two each winter, or are you preparing for a major emergency (i.e. hurricane) that kills power for two weeks? In the latter case, what is the likelihood of being able to easily obtain fuel for a generator?
What I would do is to buy a marine deep cycle battery and an inverter. (total cost maybe $200) Keep the battery plugged in to a trickle charger (and not on a concrete floor), and if the lights go out, bring the battery and inverter into the house and plug in your fridge. The battery should be able to keep the fridge cold (which requires about 1500 watts intermittently) for at least a day or two, and if it goes dead before the power is restored, you can recharge it using your car and jumper cables. The main benefit of this system is the whole system can be inside your house.
In theory, it should be possible to rig up your entire house with a semi-permanent 240v UPS. The wiring would be similar to that required for an emergency generator.
If you think that only a generator will do, I would buy a 5000 watt unit ($700) and have an electrician install a circuit ($700?)
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