Stamina and HP

Stamina is the base stat.

HP is what damage you take is applied to, it's shown on the pokemon info screen and represented by that little HP bar as well.

Stamina and level determine HP.

Every Pokémon in the game starts off with a certain number of HP, or Hit Points, when they're captured. When you use your Pokémon in a battle, any damage done to your Pokémon subtracts from the Pokémon's total number of Hit Points. So HP is kind a measure of your Pokémon's stamina and health.

As far as I understand it:

HP = Stamina
HP in gym = Stamina x2

The stamina stat is just a simple way to code it so that gym defenders can have double HP.

If anyone finds this as the top result when googling, please be aware that the answers provided in this topic are wrong. Here is the correct answer:
Max health of a Pokemon in Pokemon Go = (2*HP+HP_IV)*CPM
Where HP is the HP stat from the main series games (e.g. 91 for Dragonite), HP_IV is the IV value of your pokemon and CPM is the CP multiplier which you can find at this link:
https://pokemongo.gamepress.gg/cp-multiplier