The command below tells you how much space each directory below the current directory is using. useful in drilling down to a directory which is very large on your system

cd /
du -h -x --max-depth=1

returns

4.0K	./mnt
260K	./root
0	./sys
7.6M	./lib32
13M	./sbin
0	./dev
236K	./tmp
0	./proc
2.2G	./opt
170M	./lib
4.0K	./selinux
3.1G	./usr
15M	./etc
4.0K	./media
7.8M	./bin
16K	./lost+found
8.0K	./home
4.0K	./srv
246M	./var
27M	./boot
5.7G	.