Essential means that you absolutely cannot do without something. "Necessary" can be used to mean the same thing. However, "necessary" can also mean that something cannot be avoided. "Necessary" also means that you are forced by law to do something. Anyplace you use "essential", you can use "necessary"; however, it does not work the other way.

Yes, "need" is the keyword when talking about necessary things. "It is necessary to have things in order" = you need order. For essential, it means that what you are talking about cannot work or exist without the "essential thing". "It is essential to have things in order" = if there were no order, you could not work or function. The previous example was saying you could still work, but with difficulty. Order is the "essence", a central element. However these words are often used interchangably...

Hmm...essential implies that the object needed is the center of the event, while necessary implies that while the object is needed, it is not as important as the whole event. If too confusing, both can be used in place of each other in most situations.

is necessary your help, but you are not essential. means you are not the essence of it

Necessary means it needs to happen. Essential means it is important.