This is the shaggy parasol mushroom, Macrolepiota rhacodes.  The picture shows some of the important features of the group of fungi commonly referred to as mushrooms and toadstools. The gills (lamellae) on the underside of the cap are lined with reproductive cells that produce spores which are dispersed by air currents. Very large numbers of spores (many millions per hour) are produced but fleshy fruit bodies such as these are very short lived and soon decay. The cap of the fungus is initially egg shaped but soon expands and in this case has left a prominent ring on the stem. The material of the ring was formerly attached to edge of the cap forming a veil. Useful features in identifying fungi include:  the presence or absence of a ring; the colour of the gills and the spores; the shape and texture of the stem and cap surface.