Although I didn't see it at the time, apparently CBS Sunday Morning ran an episode on doodling last weekend. It's quite good. Seven minutes long. See it here and then read on. Well worth watching!
Doodling is an entirely separate practice from Zentangle®, but equally valid and useful. They are just used for different reasons. I've seen proponents of each getting into heated arguments over which is "better." Neither! Both are great, just different.
Here's my take on which does what. I'm a lifelong doodler, but a recent (2009) discoverer of Zentangle and only a very recent CZT (Certified Zentangle Teacher), so I do know something about each. What's below may not be 100% accurate but it's my thinking as of today.
I'm a textile artist (traditional rug hooking, punch needle rug hooking, and other textile arts), a long-time meditator, a certified meditation teacher and coach, and focused on learning about the interplay of art, creativity, and mindfulness every day.
SITES TO WATCH:
Insight Meditation Society
Oxford Rug Hooking School
Zentangle: The Official Site
Green Mountain Rug Hooking
Massachusetts Tarot Society