Social democracy is a political ideology that supports a competitive economy while also benefiting people who aren't paid a lot for their jobs. This makes sure people can support themselves in society and get more opportunities while they have them. Other factors play a key role as well, such as more regulation to prevent rich people from being greedy with the system and to bargain for a good workplace, and making the political system as fairly represented as possible. This way of governing is appreciated by people living in countries that have such a system, like the Nordic countries of Sweden, Denmark, Finland, Norway, and Iceland, and the social democratic movement has seen traction outside of those countries, as it's known for the little amount of unrest it causes and the support that the authoring political parties have.
Social democracy is a compromise, or balance, between a system that is overly capitalist, where people can own anything and competition is a lot higher, and a system that is overly communist, where everything must be shared.
Before there was a social democratic system, communism and socialism were very similar until a group of socialists called democratic socialists began to reject the principles of communism. The democratic socialists did not agree with the way the communists used violence and revolution to gain control, and advocated for peaceful reform.