Backing up multiple users account on one computer

Hi,

Just got an external hard drive and learning how to back up my data. How can I make sure the data for all users are being backed up? I share my macbook pro with my husband and child. When I attach my external HD to my mac does Time Machine automatically back up the WHOLE computer (all 3 user accounts) or does it just back up the data for the user account that's open at the time the external HD is plugged in? I'm new to this whole backing up process so any info would be greatly appreciated.

Thanks