Traditionally, solid objects are represented as meshes: a set of vertices, edges, and faces. This interpretation of real-life objects into 3D space is sufficient for typically solid media such as teapots and chairs. However, this model faces many issues when used to represent dynamic materials such as smoke or clouds, which have innumerable and constant shifts in number and location of vertices, edges, and faces. These types of objects are typically classed and implemented separately as Volumes.
Due to the nature of the materials involved, Volume rendering requires a different set of solutions in regards to data structuring, light sampling, and so on. Quickly becoming an industry standard, OpenVDB is a Volume rendering framework developed by Dreamworks that sees wide usage throughout the graphics industry such as in Pixar's RenderMan and Houdini. It is known for its efficiency in handling sparse data sets and its wide range of filters, transformation operators, compositing tools, and many other features for the creation and robust manipulation of Volumes. Its integration into Cycles will surely further Blender's usage in the 3D graphics scene.