Mediums vs. Additives
I've included additives in this page because they serve a similar function to acrylic paint mediums. The difference is that additives don't contain acrylic binders (like mediums do) so you must follow the label and use them in correct proportions. Most mediums can be added to the paint in any amount until you've the achieved the desired effect, but it's possible for additives to weaken the color and adhesion of the paint when overused.
Gloss Mediums handy if you want to enhance the natural sheen and luminosity of acrylic paint. Gloss medium will also make paint thinner and more transparent, which is useful if you want to paint thin but brilliant glazes.
Because acrylic paint is already naturally glossy, matte medium is handy when a more subtle, non-reflective finish is needed. You can also mix gloss and matte mediums together in equal parts to achieve a kind of semi-gloss.
Like gloss medium, glazing medium will make acrylic paint more transparent and facilitate subtle glazing techniques.
Glazing medium is also sometimes called glazing liquid, depending on the brand. Golden offers glazing liquid in satin and gloss sheens. Make sure you don't buy glazing medium that contains oils because it's not compatible with acrylics.
Gel medium is a white, paste-like gel that will thicken your paint so that it retains brushstrokes, which is great for impasto techniques. Gel medium also enhances the adhesive properties of the paint, which makes it suitable for collage work.
Gel mediums are available in different types, like gloss & matte, and also different textures, which I'll discuss below.
Modeling Paste, sometimes called molding paste, is like gel medium but thicker and harder. You can literally make your painting three-dimensional by building up a relief that can then be sculpted, sanded or carved once it's dry. Modeling paste is also great for collage because you can embed all kinds of objects in the surface.
Texture gels imitate certain textures, such as sand, ceramic stucco and glass beads. They're fun to experiment with and can really enhance certain artworks. You can also make your own texture by mixing in some sand, sawdust or whatever you want.
Retarding medium (also called retarders) slows down the drying time of acrylic paint so that it behaves more like oil paint. This allows you to mix colors on the support and create smooth blending effects. Most retarding mediums are actually additives (lacking acrylic binders) so follow the directions and make sure you don't use too much.
If you use blending effects a lot, it might be easier to buy Open Acrylics which are formulated to have a longer drying time straight out of the jar.
Flow improver, sometimes called "wetting agent" or "flow aid" (depending on the brand), is another additive you must use sparingly. It thins acrylic paint for use with wash techniques and painting over large areas. Acrylic paints are water-based, so you can thin them with water which breaks down the acrylic binder and makes the paint behave like watercolors. Flow improver works by breaking the surface tension of the water inside the paint which thins the paint without reducing color strength or compromising the finish.
A Few Parting Tips
Always mix in the medium after you've mixed the colors together.
Don't use oil mediums with acrylics because they aren't compatible.
Some acrylic paint medium products claim that they can also be used to varnish your paintings. I recommend you use a removable varnish as a final top coat instead of a medium.
When it comes to acrylic paint mediums, there isn't too much difference between brands (unlike paints) because most mediums and additives are made with the same chemical formula.