1. Functions can used with Select statement
   SP are not
2. If we have a syntax error or bug in SP it ignore it at
   execution. In case of Function it won't.
3. Function should return atlease one value. 
   SP may or maynot return values.
4. Function cannot include in other function.
   We can execute one SP in other.

UDF
1. Must return a value – a single result set
2. Directly used in select, order by,where, from
3. UDF can’t use nondeterministic function Such as
getdate(), rand()
4. Can’t change server enviroment variables
5. Stops execution of T-SQL code when error occurs
6. Can’t use temp table in UDF

Stored Procedure
1. Can return value – multiple result set
2. Can’t use in select
3. Can use nondeterministic functions
4. Can change server enviroment variables
5. Stored procedure move to the next instruction if you used
proper error handling,
6. Can use temp table in SP

store procedure takes less time to execute the statement 
than functions, also we can create loops inside the store 
procedure.

function does have update or delete statment.

a stored procedure is like a subroutine. It is used to 
perform tasks within the database, whether it be to INSERT, 
UPDATE, DELETE, SELECT, send return values, send output 
parameters, send e-mail, call command line arguments, 
encapsulate business logic, enforce data integrity, or any 
combination thereof. Here is a fictitious example: 
 
CREATE PROCEDURE dbo.doStuff 
    @dt SMALLDATETIME 
AS 
BEGIN 
    SET NOCOUNT ON 
 
    DECLARE     @un SYSNAME, 
        @now SMALLDATETIME 
 
    SET @un = SUSER_SNAME() 
    SET @now = GETDATE() 
 
    BEGIN TRANSACTION 
 
    INSERT dbo.myLog 
    ( 
        ProcName, 
        UserName, 
        dt 
    ) 
    SELECT 
        'doStuff', 
        @un, 
        @now 
 
    DELETE dbo.myLog 
        WHERE dt < (@now-7) 
 
    UPDATE dbo.Users 
        SET LastActivity = @now 
        WHERE UserName = @un 
 
    COMMIT TRANSACTION 
 
    SELECT TOP 3 ProcName, dt 
        FROM MyLog 
        WHERE UserName = @un 
        ORDER BY dt DESC 
 
    DECLARE @subject VARCHAR(255) 
    SET @subject = @un + ' used the doStuff procedure.' 
 
    EXEC master..xp_smtp_sendmail 
        @from = 'foo@bar.com', 
        @to = 'bar@foo.com', 
        @server = 'mail.myserver.com', 
        @subject = @subject 
 
    DECLARE @cmd VARCHAR(255) 
    SET @cmd = 'del c:\users\'+@un+'\archive\*.log' 
    EXEC master..xp_cmdshell @cmd, NO_OUTPUT 
 
    RETURN 0 
END 
GO
 
Stored procedures accept parameters, and are the preferred 
method of both manipulating data and simply returning data. 
They are compiled when first run, and the query plans are 
stored and cached by SQL Server's optimizer, and those 
cached plans are swapped out depending on frequency of 
usage. Generally, a stored procedure will perform faster 
than an ad hoc query, but there are certain cases (e.g. 
when a bad plan is cached) that this is not the case. 

User-Defined Functions (UDFs) 

In general, UDFs can be a serious source of performance 
issues. Also, UDFs cannot be used for DML operations 
(INSERT/UPDATE/DELETE), cannot use non-deterministic 
functions ,cannot use dynamic SQL, and cannot have error-
handling (e.g. RAISERROR).

FUnction use when you want to return any value only
but SP is best for all