Assuming you know the shapes and sizes of various brushes now (and what to use them for), let's wade into deeper cuts—bristle type. A very helpful tip from Tom Pecheux we heard a few years back, as he applied cream shadow to a model backstage at Derek Lam:

'You need to use a synthetic brush!” he exclaimed. “If you use a brush with a cream-based product it must be synthetic, not a natural hair. If it’s a cream blush or cream eyeshadow or lipstick or foundation, you must use all synthetic brushes. First of all, because these products are greasy, so it’s much healthier to use a synthetic, and also because synthetic brushes are flat. See, this is a natural hair brush!” He lifted one and wiggled it against the backs of our hands. “It’s fluffy!” It was. “Now, this is synthetic hair!” It wasn’t fluffy.

'With synthetic, it’s hard to do a good, fluffy brush. With cream anything, you need a synthetic!” The one he favors for cream eyeshadow? Estée Lauder's Concealer Brush. So, there you are—now you are better educated and better shadowed.