Nuclei are one of the two common forms of nerve cell organization. The other is the layered structures such as the cerebral cortex or the cerebellar cortex. The same kind of cell cluster in the peripheral nervous system is called a ganglion. Some of the traditional names for brain nuclei also use that word.
In anatomical sections, a nucleus usually shows up as a region of grey matter, often bordered by white matter. The vertebrate brain contains hundreds of nuclei, which vary in shape and size. A nucleus may have a complex internal structure, with several types of neurons arranged in clumps (subnuclei) or layers.
We have the impression of our brain working as "one thing". Actually, that's a mirage. All the parts work together so that we don't notice the way it is done. We just see a "picture of the world", and we think almost without thinking.