See the AptGet/Howto - Ubuntu Documentation page to clear your doubts.
This command removes packages that were installed by other packages and are no longer needed.
apt-get autoremove <package_name>
This command removes an installed package and dependencies.
apt-get remove <package_name>
This command removes an installed package, leaving configuration files intact.
It depends on how much you trust the dependancy tracker. While almost always correct, there are times when you would want a dependancy to remain, particularly when you are a developer or power user installing software that is not in the repository.
If you always install software through apt-get, without exception, and trust all the dependancies to be correct (which they usually are), then you can use
apt-get autoremove and gain a small amount of drive space and a reduced exposure to potential security holes by having it remove packages that no longer have any packages that need them.
But if you install software manually, or develop software, or do not want to deal with a possible dependancy error, then not using autoremove to clear potentially unused packages is probably the safer choice. Regardless of whether you use
apt-get autoremove every now and then or not, you will always remove software using
apt-get remove Package
For example, if I install
AwesomePackage, it may depend on
AwesomeLibrary, and thus
AwesomeLibrary will get automatically installed as a dependancy. When I remove
AwesomePackage using autoremove, as long as no other package has
AwesomeLibrary as a dependancy it will be uninstalled as well. But if
SuperPackage also requires
AwesomeLibrary, or if I had installed
AwesomeLibrary explicitly myself rather than having it come in automatically as a dependancy (
apt-get install AwesomeLibrary), then autoremove would not get rid of it.
The reason it is not the default is that having
AwesomeLibrary on the system, unused, is a very minor issue. It will almost never cause problems, and most dependancies don't take up much space. There are exceptions, but the times when removing a dependancy will cause problems outnumber the times when it will solve or prevent a problem.
Source: SuperUser: When would you use apt-get remove over apt-get autoremove?