Accounts receivable is listed as a current asset in the balance sheet, since it is usually convertible into cash in less than one year as it is the amount owed by the customer to the seller.
Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables.
In detail : A Foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table or the same table. In simpler words, the foreign key is defined in a second table, but it refers to the primary key or a unique key in the first table.