Both ground squirrels and tree squirrels can be a bother (some frustrated gardeners call the latter "rats with furry tails"!). Ground squirrels live in burrows but feed above ground during the day, nibbling through the garden and climbing trees to get at nuts and fruits. Tree squirrels spend most of their lives above ground, searching for nuts and acorns and burying many for winter feeding. They are attracted to bird feeders, which they like to raid; they may then nest in nearby trees and establish local colonies.
Though squirrels can drive gardeners to distraction, in many area these animals are protected from trapping or poisoning by local regulations. Check with the relevant authorities before attempting any controls. To keep tree squirrels from hiding nuts in container plantings (and digging up plants in the process), mulch the soil with decorative small rocks.