Adnan,
Here is a little more information in addition to that provided by Ben and Larry.
First of all, both applications and UBEs can be checked out/in. A get/advanced get can also be performed for both. This can only be done on a fat client. These functions are generally only available to developers and not general users.
Second, packages for UBEs have to be used to deploy them to Enterprise/application servers*. It is also easier to build a package (for both applications and UBEs) to deploy to numerous fat clients.
* There is also the option to submit specs only to a server in the batch versions application (P98305), but I not that familiar with it and it would not be as practical as packages. Others may provide more detail on this and/or you can do a search on JDEList, Oracle, Quest and other websites for more information.