What's the difference among tablet, pill and capsule?

Normally, tablet is a small flat piece of medicine, can be any shape. But pill is a ball or disc of solid medicine. And capsule is easy to tell, it is the medicine with a soluble case.

Pill:A small pellet or tablet of medicine, often coated, taken by swallowing whole or by chewing.
He took a few pills before going to bed.
Tablet:A small round solid piece of medicine. A slab or plaque, as of stone or ivory, with a surface that is intended for or bears an inscription.
The doctor told me to take two tablets before every meal.
capsule:An outer covering containing a measured amount of medicine, the whole of which is swallowed.(装入某一剂量药物以便吞服的）胶囊
The doctor advised me to take a capsule this morning.

简单理解就是 pill 是指外面涂上一层的那种药 如 糖衣片 Tablet 是那种大白片的药 像止痛片那样的 Capsule 就是胶囊啦 很好理解。

tablet [简明英汉词典]
[5tAblit]
n.写字板, 书写板, 碑, 牌匾, 拍纸簿, 便笺簿, 药片, 小块

pill [简明英汉词典]
[pil]
n.药丸, 口服避孕药, 弹丸

capsule [简明英汉词典]
[5kApsju:l]