Vacuoles vary in function between organisms and even within the same cell, but the most frequent use is the storage of water or nutrients. A vacuole is really a general-purpose, empty membrane organelle that is filled with whatever the cell needs to keep separate from the rest of the cytoplasm. As such, different vacuoles also store waste or even toxins, and some organisms have other uses for them.
According to the City University of New York, plant cells are particularly dependent upon their vacuoles, specifically a large central vacuole. This large vacuole, which often takes up a large majority of the volume of the cell, is filled mostly with water, but also waste and other substances. The remarkable thing about this vacuole is its structural role, however. Plant cells lack the protein-based cytoskeleton that helps animal cells keep their shapes. Instead, they use the pressure of their inflated central vacuoles against their tough cell walls to maintain their shape, in a manner analogous to a Mylar balloon.
Since vacuole is actually a general term for a hollow, membrane-bound organelle, the name also applies to the rather different organelles known as contractile vacuoles. These are found in many single-celled organisms, and help maintain water balance by physically pumping excess water from the interior of the cell.