ANSWER - This used to really confuse me too! I remember once telling a far more experienced witch that I was going to make a tincture from boiling water and herbs and she sent me quite a pitying email with the polite suggestion I do some research first… Lol!
Simply put, a tincture is a solution where the main ingredient (usually herbs) is steeped in alcohol to produce a concentrated liquid solution of the herb. The alcohol absorbs the flavour/scent/property of the herbs and is a useful way of preserving the herbs for future use - alcohol is a very effective preserving agent. Alcohol is an excellent choice for a base dilute if the tincture is intended to be consumed. For a non consumable tincture (perhaps one intended for spell work rather than for drinking) other substances (most popularly and far more cheaply) like vinegar can be used.
A tisane is very similar to a tincture but instead of herbs being infused in alcohol they are infused in heated water. The simplest example of a tisane is a plain old cup of tea.
An infusion is a generic term for both the definitions above but tends to be most commonly used in witchcraft to refer to an infusion of herbs in oil. Oil infusions are rarely for consumption (though they can be – think chillis in olive oil) but they can be used for anointing, dressing candles or even for body massage. Infusing herbs in oil will make them go further as the oil brings out the potency of the her.
Just to confuse things further there is also decoction which is the method of boiling a substance to extract a substance from it. This tends to be used for harder substances like tree bark or roots (substances that wouldn’t yield much if just steeped in fluid). Decoction involves breaking up the substance and then boiling the broken pieces up into a concentrated liquid.
The term potion is less easily defined as different witches will use it in different ways and the term can be quite subjective. The dictionary defines it as “A drink or draft, especially one having or reputed to have medicinal, poisonous or magical powers.” So with this definition a tincture, a tisane or an infusion could all be considered to be a potion. I’d personally widen the definition of potion to also include liquid solutions not intended for consumption. I’d include in my own definition of a potion a liquid I made up to pour across a boundary, a liquid I created to anoint a person or poppet or a liquid I made for the purpose of immersing a taglock.