Poe Soul

After a Poe is defeated, all that remains of the monster (if anything) is its Poe Soul.

Ocarina of Time
In Ocarina of Time, a defeated Poe will drop its lamp onto the ground, revealing a Poe Soul trapped inside it. One of these can be snatched up in an Empty Bottle, but if left untouched for a dozen-or-so seconds, it will disappear. A trapped soul can be sold to the Collector near Hyrule Castle Town for 10 Rupees, or consumed. A consumed soul could either restore a part of Link's health or take away health until he only has a partial Heart Container remaining.

Rarer Big Poes drop more valuable souls the Collector will pay a larger sums for, but the consumption effects of these souls appear the same.

Twilight Princess
In Twilight Princess, Link can collect Poe Souls to receive various rewards from Jovani. There are 60 Poes in all spread around Hyrule. Poes can only be seen and defeated with Wolf Link's senses, so they cannot be obtained while Link is in his regular, humanoid form. Souls are now stored automatically in any amount on the menu's subscreen, and don't need bottles like they did in Ocarina of Time.

Collecting twenty souls for Jovani will net Link a Bottle of Fairy Tears. With sixty souls Jovani will give Link 200 Rupees whenever he asks. This method of collecting the remains of dead enemies for rewards from cursed individuals is vastly similar to the Skulltula Tokens minigame in Ocarina of Time.

See a list of all Poe Soul Locations at StrategyWiki