Volcanic tuffs have the same origin as volcanic breccias - explosive volcanic eruptions. Volcanic tuffs lack the abundance of large, angular grains present in volcanic breccias. Tuffs are fine-grained to very fine-grained ash deposits that have become lithified. The degree of lithification seen in tuffs varies. The sample shown below is very loosely consolidated - individual grains can be easily separated from the rest of the rock.
Some ash beds remained so hot after initial deposition that the heat was sufficient to self-solidify the beds. Such deposits are called welded tuffs. Some welded tuffs have glassy cores, where the ash was heated past the melting point after deposition (for example, see the famous Resting Spring Pass Tuff east of Shoshone, eastern California, USA)