What is the difference between sentirse and sentir?

Both sentirse and sentir mean to feel, but aren't used interchangeably. Sentirse is used to describe how you feel, and is followed by and adjective (e.g. feliz, triste). Sentir is used to describe what you feel, and is followed by a noun (e.g. felicidad, tristeza).
ExamplesNo me siento bien - I don't feel wellSe sienten tranquila - they feel calm
Sentía rabia - I felt ragePodía sentir el sol - I could feel the sun