What is the difference between 需 and 須?
The answer to this question: What is the difference between 必须, 得, and 需要? already explained the difference between 需 and 須. 

Basically:

需(need) apply to noun. For example: "必需品" (品 is a noun)

須(need to; must) apply to verb. For example: "須知人言可畏" (知 is a verb)

必需品: necessity; essential thing

人言可畏 criticisms should be feared; gossip is a fearful thing