求助！这个@@IDENTITY是什么东西？

[5200]

我是新手。
存储过程如下

ALTER PROCEDURE sp_Accounts_CreateUser
@EmailAddress varchar(255),
@Password binary(20),
@FirstName varchar(30),
@LastName varchar(30),
@Address1 varchar(80),
@Address2 varchar(80),
@City varchar(40),
@State varchar(2),
@ZipCode varchar(10),
@HomePhone varchar(14),
@Country varchar(50),
@UserID int output
AS
INSERT INTO Accounts_Users(EmailAddress, Password, FirstName, LastName, Address1, Address2,
City, State, ZipCode, HomePhone, Country)
VALUES(@EmailAddress, @Password, @FirstName, @LastName, @Address1, @Address2,
@City, @State, @ZipCode, @HomePhone, @Country)

SET @UserID = @@IDENTITY

RETURN 1


请问那个@@IDENTITY 是拿来做什么的
那个SET @UserID = @@IDENTITY　又有何含意
还有最后的Return 1　是什么意思？？？
每题３０分！在线等！