Something that's attached is connected to something else. If your rain coat has an attached hood, for example, it's fastened to the coat.
A child might stay attached to his mom on the first day of school, clutching her, or your cat might remain attached by his claws to the sofa when you try to pick him up. When an extra room is connected to your house, you can call it attached as well: garages are often attached. You can also be figuratively attached, or deeply adoring of someone or something: "I'm pretty attached to my stuffed animal collection."