Flasks are used to store health (red) and mana (blue) potions which can restore a character's health and mana respectively when consumed. Currently, each character can carry up to five flasks that can be used by the corresponding 1-5 keys. Different sized belts may allow the character to carry more or less flasks, though.
Flasks can hold multiple charges with each use depleting a single charge. A flask will visibly empty when it is used to provide a clear approximation of how many charges remain. Increasingly larger flasks which hold more charges can be found as the character progresses farther through the game.
Flasks held in a character's belt will regenerate charges automatically as a character kills monsters. Flasks in the inventory cannot store charges and any flasks moved from the belt to the inventory will lose any charges they are holding. flasks can be filled at wells.
Size: Flasks come in several different sizes which correspond to how many charges the flask can hold and how much health or mana it restores.
Life/Mana Recovery: The main purpose of flasks. The amount a flask can restore will vary greatly but roughly corresponds to the size of the flask. Larger flasks will generally restore more.
Recovery Time: How long it takes to recover the health/mana amount listed. Recovery is continuous and constant over this period of time. A base time of three seconds seems to be most regularly used but this will most likely vary considerably from flask to flask.
Charges: How many drinks can be taken from the flask before it is empty. Minor, medium, and large flasks can hold three, four, and five charges (or equivalent) respectively.
Each Drink Uses: How many charges each drink uses. The majority of flasks will use one charge per drink, however, some may use more. Typically, if a flask uses more charges per drink than it will hold more charges to compensate.
Like other equipment, flasks come in different rarities including normal, magic, and unique. Magic flasks will feature up to two affixes which can augment or modify the effects of the flask. Unique flasks are hand-crafted by the developers and will often feature unique magical affects.
The bonuses granted by flask affixes are only triggered when the flask is used and last only for the duration of the recovery time. Generally, prefixes modify the characteristics of the flask itself while suffixes add additional effects beyond health/mana recovery.
Ample: Increased charges.
Caustic: Increased recovery but at the cost of a health/mana reduction.
Panicked: Instant recovery below a certain percentage of health/mana.
Animation: Percentage of recovery goes to minions as well.