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:
/data/data/org.foo.bar.app/
|
+- databases/
|
+- lib/
|
+- shared_prefs/
|
+- cache/
|
+- files/
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!