Determining used disk space

I have a simple newbie question:

What command or program will tell you how much hard disk space is used and
how much remains?

Thanks.

Reply to:

Follow-Ups:
- Re: Determining used disk space
  - From: Travis Crump <pretzalz@techhouse.org>
- Re: Determining used disk space
  - From: Chris Kenrick <chrisk@aurema.com>
- Re: Determining used disk space
  - From: Vineet Kumar <debian-user@virtual.doorstop.net>
- Re: Determining used disk space
  - From: Mike Mimic <ppagee@yahoo.com>
- Re: Determining used disk space
  - From: Patrick Wiseman <pwiseman@mindspring.com>
- Re: Determining used disk space
  - From: Larry Smith <doccpu@yahoo.com>
- Re: Determining used disk space
  - From: Rob Weir <rweir@softhome.net>
- Re: Determining used disk space
  - From: ernst-magne <ernst@bluezone.no>