Vacuoles are surrounded by membranes. They are sort of like a storage bubble in the cytoplasm. Vacuoles in animal cells are considerably smaller than those in plant cells. In animal cells vacuoles may store food that needs to be digested. Food cannot pass through membranes until it is broken into smaller particles. The lysosome can fuse with the vacuole to break down what is in there. Your white blood cells do this when they eat invading bacteria. Vacuoles can also store the undigestible wastes until they can fuse with the cell membrane and squirt the wastes outside.
Vacuoles in animal cells can form when the cell membrane surrounds a material and pinches off to bring the substance inside the cell. This process is called endocytosis.