1. User defined functions can be used in select statements. 
Procedures can not be used.

2. Functions must return some value. Procedures do not 
always return the value.

3. Functions will take only input parameters. Procedures 
can take input as well as output parameters.

4. Not all types of user-defined functions use 
INSERT,UPDATE,DELETE statements(Only multi-statement table 
valued functions can use). All procedures can use these 
statements.

1.A stored procedure is a precompiled collection of 
Transact-SQL statements stored under a name and processed 
as a unit that you can call from within another Transact-
SQL statement or from the client applications.
2.Stored procedure performs an action.
3.A collection of SQL statements often combined with 
Control-of-flow statements.

1.User Defined Functions are compact pieces of Transact SQL 
code, which can accept parameters, and return either a 
value, or a table. They are saved as individual work units, 
and are created using standard SQL commands. Data 
transformation and reference value retrieval are common 
uses for functions.

1. function always run with select command while stored 
procedure execute with execute command

2. function not provide output parameter facility
 while stored procedure provide output facility

3. function always execute means it execute more than one 
time while store procedure execute only one time

Userdefined function can used in select statement.

It must return a values but in store procedure it is 
optional.

Store procedure can use using sqlparameter but in 
userdefined function it is not used.