Nautilus is the default file explorer used in Ubuntu. Sometimes, you may need to kill and restart it to see the effect of installing or removing a program.
The easiest way to kill Nautilus is:
$ nautilus -q
You can restart it after this. If that does not work, you may need to log out and log back in. But, this is almost never needed.
Tried with: Nautilus 3.4.2 and Ubuntu 12.04 LTS