In Sql Server Function are Like S.P Mean Batch of SQL 
command. But Function Must Return Single Value
In SQL Server Four Type Of Function
1)Table-Valued Function:Return Datatable
2)Scaler Valued Function:return Single value
3)Aggreated Funtion:Return Aggreatged Result set
4)System Funtion:used by system like Rowset etc.

CREATE FUNCTION function_name
(@input_variables  type)
RETURNS data_type of result returned by function
AS
BEGIN
.....  SQL Statements
    RETURN (data_value)
END

Let's demonstrate this statement by writing a function.
Oracle has a useful function called NEXT_DAY that computes
the date of the next named day of the week (i.e., Sunday,
Monday, etc.) after the given date. Go ahead and write your
own function for SQL Server (see Listing 1 for the code to
create a scalar function using NEXT_DAY).

Inline Table-Valued Functions
Now create a function that returns a table. Use the
Northwind database; it contains customers, orders, and order
details. Suppose you want to analyze your customers by
ranking them from best to worst. The function that you write
will return the customerid, total sales, and rank of each
customer in a table that can then be used by itself or
joined within other tables. Furthermore, you will pass a
parameter to the function to filter the customers based on
the rank (e.g., top 3, top 10, etc.).

First, create a view in the Northwind database that
summarizes the total sales for each customer, as follows:


use Northwind
go
 if exists(select * from sysobjects where 
   name = 'vw_totalsales')
   drop view vw_totalsales
go
create view vw_totalsales
as
(select CustomerID,sum (UnitPrice * Quantity * 
   1 - Discount) as 'totalsales'
from Orders o 
inner join [Order Details] od
on (o.OrderID = od.OrderID)
group by Customerid )

To find the rank of each customer, you need to find the
number of distinct total sales values that are equal to or
less than the customer in question. See Listing 2 for the
code. Look more closely at the syntax you used. First of
all, in this function you stated "returns table." That tells
SQL Server that the function is returning a table of results
rather than a scalar value. But you didn't define the table
of results the function would be returning. Instead, you
followed it with a single return statement that has one huge
SELECT statement in it.