I found this to be so helpful that I thought I'd share...

Great Blog Post from Ulta.com - Choose Your Tools, Synthetic vs. Natural brushes

I must admit, I've never given much thought to the difference in using natural or synthetic makeup brushes.  I've always decided on what brushes worked best for me by using the old "trial and error" method.  Ulta, one of my favorite retailers, has a great quick read on which type of brush to use for certain applications.  You can read the post here:  Choose Your Tools- Synthetic vs Natural Makeup Brushes