Acrylic medium is the same formulation as acrylic paint, minus the pigment. It can be used as a paint additive, an adhesive, and a texturizer. It also can be used to create image transfers. It comes in three basic weights (medium, gel, and paste) and two finishes (matte and gloss). The different weights and finishes are inter-mixable, so you if you cannot find exactly what you need, or you are on a budget, you can create it using two or more products.
Medium is the thinnest of the acrylic mediums. It is easily pourable, with a consistency like that of cream. It works well as an adhesive and for creating image transfers. It can also be added directly to acrylic paints, to extend them or to alter the consistency, transparency, and finish.
Gel is thicker than medium. It is pourable, with a consistency like that of thin yogurt or kiefer. It works well as an adhesive and for creating image transfers. Gel can also be added directly to paint to alter consistency and transparency. It can also be applied to the surface prior to painting, to create texture.
Paste is the thickest of the acrylic mediums. It is too thick to pour, with a consistency like pudding. Depending on the manufacturer, it may be called either molding paste or modeling paste. While mediums and gels dry clear, paste may contain fillers that make it opaque. Paste is a good way to build up surface texture, prior to painting. It can also be mixed with acrylic paints, though be aware that its opacity will alter the look of the paint.