The electricity sector enjoyed a regulated monopoly in many countries. In such cases, vertically integrated companies are or were responsible for supply, generation, transmission, distribution and supply of electricity.
In Europe, policymakers have triggered competition since 1996. Electricity suppliers and generators can freely establish themselves (subject to meeting authorization requirements) and they can compete among each other. Unbundling is the (regulatory) norm now: generation, transmission and distribution need to take place by different players. Also, companies need to respect the open connectivity nature: for instance, a transmission company cannot forbid that a distribution company would connect to its infrastructure, purely based on will. Still, transmission and sometimes also distribution is in many countries still organized as regulated (natural) monopolies.