Herbs can be preserved in a variety of ways for use at a later date, thus allowing you to use herbs that may not be freshly available. How the medicinal properties of herbs are extracted and preserved are dependent on the solvent (liquid) used, and what purpose that the herb will be used for. For examples, alcohols can extract resins from plants, and water is used for mucilages.
Tinctures are created typically with alcohol or glycerin as its solvent, have a long shelf life, and are taken by the drop. Alcohols and glycerins are also useful for extracting certain components of an herb that may not dissolve in water (such as when you make an herbal tea).