Gloss mediums and matte mediums are shiny or matte, clear, acrylic finishes, and can be also used for glazing. I mostly use water for glazing. The most noticeable difference between using water and gloss medium is that your glaze may take on the "property" or sheen of the medium. A gloss medium will make your glaze shiny, and a matte medium will make it look flatter. It can be seen as an annoyance, or you can use this to your advantage if there is a specific look you are going for in a painting or in parts of a painting. Water is neutral and won't change the properties of the paint you've used. 

There are specific mediums made for glazing called "glazing mediums or liquids" and they differ from regular gloss or matte mediums in that they have a retarder in them to slow the drying of the paint. This allows the acrylic to stay "open" so it can be worked longer. This can be helpful when glazing very large areas on bigger paintings. For our purposes, with the 12x12 panels we're working on, mixing with water does the job and allows for a quick dry time as well, so we can continue working on the pieces.