Deleting Apps : how to free memory space

Hello,
Deleting the downloaded apps is easy on an iPhone. However, the user guide says;

+"When you delete an application, its data is no longer accessible via the iPhone user interface, but it isn’t erased from your iPhone."+

So,
1) Why doesn't it erase the application?
2) Is there a way to regain this space from the memory?
3) Can the deleted apps fill up my memory space to the max after many install&deletes?
4) Does it store every version of each application as well?


Best,
Onur.