VanDeGraph of youtube recently put up an excellent video explaining Spain’s autonomous regions.
He does a very good job of explaining the crucial distinctions between between federal and unitary states[1], and why Spain, despite its very high degree of decentralization, is not (strictly speaking) federal – and, by implication, why some countries which do not actually call themselves federal probably are (e.g. South Africa).
[1] I do, however, disagree with VanDeGraph’s distinction between federations and confederations as hinging on the right to secede, or that federalism necessarily excludes this right.