都表示必需，需要之意区别
1. essential 表示语气更强力，至关重要，表示没有不行的含义
- It was absolutely essential to separate crops from the areas that
animals used as pasture.
- The apartment contained the basic essentials for bachelor
life.
2. necessary 表示需要，意思和need相近，但是没有到达没有不行的地步
- I kept the engine running because it might be necessary to leave
fast.
- Scientific work is differentiated from art by its necessary
connection with the idea of progress.
引用出处：
http://www.differencebetween.net/language/words-language/difference-between-essential-and-necessary/
有道词典
同义词解析 (essential necessary),布布扣,bubuko.com
同义词解析 (essential necessary)
原文：http://www.cnblogs.com/myprogram/p/3775277.html
