In the UK a biscuit is a small, flat, baked cake, either sweet or plain. A cookie is the American term, usually used for sweet ones. I have seen "biscuit" used in America for a form of bakery called a Scone in the UK.
A Garden is generally the cultivated piece of ground around a domestic building, wheareas a Park is a large area of ground, which may have woods and fields, attached to a large country house. Urban parks are more formal than these, and can include gardens.