Particles are a collection of attribute buffers that can be used in order to sequence compute shaders. They are isomorphic to Mesh in the sense that they contain attributes and sets of data. In this way, you can think of a Mesh as the CPU representation of a Particles object, and the Particles object as the GPU representation of a Mesh. This allows convenient initialization of particle simulations from existing meshes, or using a mesh as a constraint for a particle simulation, like a volume or a surface.

Another way to consider particles would be as the compute equivalent of Graphics. Where the Grpahics object allows you to issue high level rasterization commands, the Particles object allows you to issue high level compute commands. In this way, you can think of a Particles object as a compute shader that is executed on the GPU, and the attributes as the inputs and outputs of the compute shader. In practice, a compute shader may also require additional data, such as textures or bound vertex buffers, but the Particles object provides a high level abstraction for sequencing compute shaders and managing their inputs and outputs.

Of course, it's not very interesting just to have a collection of attribute buffers on the GPU. The real power of Particles comes from the ability to rasterize them. This is done using the particles function, which takes a Particles object and a Geometry object as input, and issues a draw call that rasterizes the particles using the current drawing state. In other words, it instances the provided geometry over the particles, using the attributes as to determine each instance's position, orientation, and other properties. This provides a powerful bridge between Particles and the rest of the drawing API.