Squirrels are small, furry rodents whose natural habitat is arboreal. In nature, they live in trees (mainly hollow ones) and make their livings finding and storing nuts and seeds. Toward the fall, they become very active as they harvest newly-fallen acorns and other nuts. They eat voraciously to put on weight for the winter, but they also hoard and store nuts away.
Another thing squirrels do in the late summer and early fall is look for a warm place to spend the winter. Hollow trees may be okay, but the attics of houses and other human-occupied buildings are prime real estate for squirrels. I mean, seriously, if you had the choice of a damp, hollow tree or a nice, dry, comfy attic, which would you choose?
Both adult and older juvenile squirrels look for new homes in the fall. Younger juveniles usually spend their first winters with their mothers, but weaned squirrels often set out on their own (usually with a few buddies of the same age who may or may not be litter-mates) and spend their first winters together, away from their mothers.
Squirrels make a huge mess once they get into your attic. They gnaw at anything they can get their teeth on; create a health hazard and odor situation due to their droppings, urine, and shed parasites; and can cause fires or outages when they gnaw through electrical wiring or telephone, network, or cable TV lines. Most of the time they also cause damage to the building itself when they chew their way in. This can also make the house more vulnerable to water damage or to infestation by other animals.
The key to lasting squirrel control is squirrel exclusion, or "squirrel-proofing." This means sealing up the house so squirrels can't get in. As you might imagine, it's easier said than done. Squirrels have streamlined bodies that allow them to get into fairly small holes. Basically, if they can get their heads through a hole, they probably can squeeze the rest of themselves through.
In addition, squirrels are blessed with sharp teeth and powerful jaws that they use to gnaw their way into buildings when they can't find a hole that's large enough. Usually they start with a small gap or hole and then just chew away at it until it's big enough for their liking. They can chew through most building materials if they need to including wood, shingles, aluminum or vinyl siding, and even metal flashing if they have to.