Nodes are used in the Node Editor for compositing, post-processing, creating materials and textures. Use this tag for anything node related.

Blender's Node Editor can currently use three types of nodes, Composite Nodes, Material Nodes (for both BI and Cycles), and Texture Nodes

There is also currently development on nodes for Particles, as well as some mention of creating nodes for other parts of Blender in the future as well.