We use callouts to call webservices, while Outbound messaging allows you to specify that changes to fields within Salesforce can cause messages with field values to be sent to designated external servers.
I would suggest you to go through following help link for a better understanding of these concepts:
We use callouts to call webservices, while Outbound messaging allows you to specify that changes to fields within Salesforce can cause messages with field values to be sent to designated external servers.
I would suggest you to go through following help link for a better understanding of these concepts:
http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_om_outboundmessaging.htm
https://developer.salesforce.com/forums/ForumsMain?id=906F000000097doIAA
http://wiki.developerforce.com/page/Apex_Web_Services_and_Callouts
http://wiki.developerforce.com/page/Apex_Callouts