你要想对一个事物知道的很详细，你就必须了解它的各种细节。

详细是形容词(adjective)，例如： 你能把这件事说的详细点吗？细节是名词（noun）例如：我想知道这件事的细节

详细——usually means some content, eg:你可以详细的说一下吗？ 细节——usually means some key points. eg:说一下具体的细节

Dear Jeff! Thank you for your best answer.

Besides, the given answers, there is also a huge difference... 详细 = detailed 细节 = meticulous, the care and thoughtfulness in the way in doing things. For example, if you visit a sick friend and bring some food, you should be aware the kinds of food you bring. It may not be good for the person even though it may be his favorite food. This sort of seemingly unimportant thoughtfulness is 细节 and is not interchangeable with 详细.