A glossy button is the false separation that splits a button into a top and bottom section effectively displaying a pronounced 3D texture. The glossy buttons are extensively used on websites / design layouts as a call to action and while they are not entirely fazed out, they are tired and somewhat overdone.
Instead, I prefer a button that is clean and cohesive in appearance. In the sample below, you will see how the gloss and non gloss versions of the button differ in user experience.
While both are still applicable, the trend towards the cleaner non gloss button is the approach to pursue in website and graphic design.