The following excerpt from the 2009 article by Sandy Bartholomew, CZT and author of the very popular Zentangle books and , explains the difference between Zentangle and doodling:
Zentangles do not contain recognizable objects and there is no “right side up”. The Zentangle method is very focused and mindful, whereas doodling is generally something you do with your hands while your thoughts are occupied with something else. It’s easy to confuse the outcome of Zentangle with doodling, but they are quite different processes.
A friend gave me a Zentangle kit and I’m afraid to add yet another fun time suck to my life. Because I am a time-suck fanatic, doing ANYTHING but what i should be doing. I just put it on the shelf to pick up when I’ve done what i need to do. If I ever do those things. 😉