What is the difference between philosophers and natural philosophers?

The difference between them are a bit vague, but basically natural philosopher is another earlier term of a scientists who wants to understand the world with empirical methods, while the philosopher is much more broader term that has to do with philosophers dealing with ethics, political philosophy, metaphysics, and epistemology. Now note that the "Natural Philosopher" is an outdated term that is no longer used now, the terms existed when there was increasing competition among schools of thoughts on the nature of the natural world. When science became more uniformed and organized since Newton and Galileo the term Scientist is much more appropriate.