"Tinctures are concentrated liquid extracts made with fresh or dried herbs. Herbal material is macerated (soaked) in a natural solvent called a menstruum. The menstruum -- usually a mixture of distilled spirits and water -- dissolves and carries the active constituents out of the fibrous plant material...
"The key advantages to tinctures are that they are concentrated and convenient to use, easy to mix into combinations, dosages are easier to control, and they have an excellent shelf life. They may be taken internally, applied directly to the skin, or used in external applications like fomentations."
(excerpt taken from book, Practical Herbalism, by Philip Fritchey, Whitman Productions 2004)
Tinctures are most commonly made with alcohol as a preservative. At the time of writing this, I have a red raspberry leaf, nettles, and peppermint tincture in vodka sitting in my kitchen cupboard, half-made. Vodka is cheap and tastes mild, which is why I prefer it. Other alcohols may be just as easily used. Rum, wine, brandy. Remember that Christ Jesus on the cross was offered bitter herbs in wine and initially refused them? Herbal infused wine was a common medicine at the time, and he was very likely being offered pain relief, sedative, dulled awareness, even perhaps something to help him die faster, which may not have been desirable to him while he was bearing the full weight of our sins. I can also think of many historical stories in books I read as a child of Medieval castles under siege, wounded knights biting down on herbal-wine-soaked rags whilst having their shining armor removed to reveal ugly battle wounds, or stories of Viking warriors pulling out a flask of herbs and wine to give to their dying, skull-cracked friends or pouring it over their own wounds, or of Irish witches lacing herbs into cups of grogg, which is spiced or "mulled" wine, in order to gain control over the tribal kings... Herbs have long been preserved in alcohol for potent use and convenient shelf life. Herbology was for the longest portion of history the only form of medicine known or applied. It is only in the last century that we have the become enraptured with synthetic medicines and have forgotten the vast store of cures present in natural sources.
Some people dislike tinctures because of the alcohol base. Let me point out that there are reasons for using alcohol, and also ways around it if alcohol is still undesirable in the circumstances. Pregnant or nursing women, for example, typically don't want to regularly consume a tincture containing alcohol. They could add their tincture to a hot drink, instead, and allow the alcohol to evaporate off with the steam for a few minutes before drinking. That's a pretty easy fix. The residual taste of vodka in the drink is hardly noticeable. Or, they could make their tincture using 5% or more acidity apple cider vinegar. The acidity is important for shelf stability of the final product. I recently came across a brilliant solution for giving children an immuno-stimulant tincture, of echinacea preserved in honey. The honey is not then useful in dropper form, as most tinctures would be measured, but a teaspoonful of herbal infused honey directly in the mouth is really not that bad! Honey would be a solution better for hard herbs than for soft, and only for consumable tinctures rather than ones that are made for external application. Vegetable glycerine may also be used as a shelf-stable, preserving tincture base, and is the most commonly used in children's tincture products. Like honey, it often goes down easily due to the sweet taste, but is still easily measured by standard droppers.
So as you see, there are alternatives. However, alcohol does retain one particular benefit that honey or vinegars cannot offer. Alcohol is very quickly assimilated into the blood stream and taken into the liver, whereas water or vinegar tend to "roll off" the liver. In general, this fast absorption means that stomach acid is a lesser hazard to consumable alcoholic tinctures, and herbal qualities desired by the user are more rapidly taken to the point of complaint or need in the body. We tend to abuse our livers. We feed them full of toxins, as they cleanse our blood and bodies from waste we cannot otherwise rid ourselves of very quickly. This isn't just the big stuff, the alcoholism, smoke abuse, or the person who requires a lot of different medications for controlling severe health problems, but the small stuff that we use daily or frequently without thinking. Non-organic produce with insecticide residues, the metals contained in your antiperspirant deodorant, the paracetemol you keep on hand for all the little headaches, the ibuprofen you use every month during period pains. Some herbs are especially beneficial for ridding the body of the damages these goods leave in their wake, and cleansing the liver from clogging toxins often helps the rest of the body to work more efficiently as the blood is cleaner and happier.
Here is a wonderful video, made by the delightfully cheerful Shoshanna of Bulk Herb Store, that provides not only instruction for general tincture making but also information about a liver cleansing tincture.