That depends, a lot of developers do not properly take care of their associated application's data and may leave them lying around. The Package Manager uninstalls the apps from
/data/data/..../ depending on the package name.
For example, Application MyFooBarApp whose package name is
org.foo.bar.app then this would be the spot -
/data/data/org.foo.bar.app/ which would have the following:
Those five sub-directories are standard placeholders for the apps to function. And Package Manager will only remove those files and sub-directories upon un-install of said application.
All too often, out of hindsight or bad code, the apps would not automatically clean up after themselves, such as placing temporary files on the
/sdcard or heavens-forbid, in
/data/local/tmp which incidentally, is the worst abuse of it as it is world writeable!