- Kind of, it backs up constantly as long as the drive is connected.
Hourly, not constantly
Every change, every hour
Following the initial backup of your entire Mac, Time Machine automatically makes incremental backups every hour, every day, copying just the files that have changed since your last backup. And it does this all in the background, so you can continue working while Time Machine is busy copying your files. Time Machine saves the hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month.
http://www.apple.com/macosx/what-is-macosx/time-machine.html