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.