Ones made from synthetic fibres and the other natural right? Well yes, but there's a lot more to it than that.
When choosing brushes think about what you're going to be using them for. Generally speaking natural brushes work best with powdery textures such as eyeshadow, finishing powders, blush and bronzes. Synthetic fibre brushes work best with cream textures including foundations, concealers, lipsticks/gloss, liquid eyeliner, cream eyeshadow & blushes.
Quality brushes make all the difference when applying makeup but it doesn't necessarily mean they need to be expensive. Ensure the brushes don't absorb a lot of your product and they don't excessively shed when using them.
Some natural fibre brushes may cause a reaction if you or your client are allergic to animal hair. They can be made from goat, hoarse, squirrel or sable hair so always test first!