Difference between User exits & Customer exits :

1.  A user exit is considered a modification, since technically objects in SAP namespace are modified. Customer exits do not affect standard SAP source code.
2. User Exits are forms (in the SAP Standard Includes) and are called by SAP standard programs using perform. Customer Exits are functions so they are called using call function (or more exactly Call customer function). 
3. Inside the form (user exit) you can read and change almost any global data from host program. Inside a function (customer exit) you can only access your import/export/changing/tables parameters. 
4. User exits are more flexible because you have more information to use in your code but on the other hand, it is very easy to manipulate erroneously global data and lead the standard program to a dump or even to make database inconsistent. Customer exits are more restrictive but you are sure any change you can make to any parameters will never lead to inconsistency. 
5. User-exit doesn't have any classification. In customer-exit we have function-module exit, screen exit, menu exit. 
6. User exits are basically designed For SD module. Customer exits are available for MM, SD, FI, HR... basically designed for all modules. 
7. While changing User-exit, Access Key is required, whereas in Customer-exit no access key is needed.
8. User exits are flexible and inconsistent and customer exits are restrictive and consistent. 
9. User-Exit affects modifies the standard code and this change will be effected every time. Whereas Customer-Exit will be included (or called) into the main program only when the corresponding Project is active. Thus preserving us lot of resources time.