Socialism is a form of government and economy where the government runs all the businesses.
Socialism used to be the economic system of the Soviet Union, who were our implacable enemies and were really quite a threat to us. Because of that, some people see it as verging on evil.
Socialism roots in the scripts of Thomas Moorus (Saint Thomas). It was then developed during the French revolution, got later several national "branches".
Marx and Engels 'adjusted' socialism towards communism.
The communism in the USSR was called "real existing socialism" = their description of communism.
In European countries, we still have quite a few "socialist" governments, by the democratic kind of socialism, called "social democratic".
Many churches follow another kind of socialism, called "Christian socialism".
The economic system which we have got after the 2nd world war by the conservatives is called 'social market economy', which is a mixture of a bit capitalism with a lot of socialism.
I don't think that you can explain socialism in a few sentences.