A squirrel is a small rodent with a large bushy tail, found in most countries of the world. It is a member of the order Rodentia, family Sciuridae.
One well-known trait of the squirrel is the gathering and storing of nuts for the winter. Squirrels will gather nuts and store them in any accessible hiding place or will bury them almost anywhere.
Squirrels are generally clever and persistent animals; in residential neighborhoods they are notorious for eating out of bird feeders, digging in potted plants either to bury or recover food, and for setting up house in sheltered areas including attics. Squirrels are sometimes also pests because they chew on various edible and inedible objects; the habit helps keep the squirrel's teeth sharp and also wears the teeth down (rodents' teeth grow constantly). Many companies sell bird feeders which are supposedly "squirrel-proof"; most of them in fact are not. Homeowners in areas with a heavy squirrel population must keep attics and basements carefully sealed to prevent property damage caused by nesting squirrels. Fake owls and scarecrows are generally ignored by the animals, and the best way to prevent chewing on an object is to coat it with something to make it undesirable: for instance a soft cloth or hot sauce.
Public domain picture from U.S. Fish and Wildlife Service
The red squirrel population in Britain has declined in recent decades, because of habitat loss and competition from gray squirrels. Recent conservation efforts include preserving and planting the conifer forests that red squirrels prefer. (See http://www.wildlifetrust.org.uk/durham/RedAlert/)
For the animal commonly known as the ground squirrel, see Chipmunk.