These are so easy and inexpensive to make, you may want to have one in every colour! Red restores some hearts, blue gives you full health, green will restore your magic gauge to full, or replenish your stamina. And then there are the less common ones, only available in a few games, such as purple Chu jelly in Twilight Princess, which generates a random effect. Check out Zelda Wiki's potion page for more details.