Socialism is the idea that minimizing human suffering and maximizing human happiness, over the long term, should be the primary goals of any economic system – in other words, that society exists primarily for these goals, and economic systems should be evaluated in terms of how well those goals are served.
It is not itself an economic system, but more of an heuristic by which to evaluate economic systems.
Such evaluation does tend to lead to systems which may place upper limits on personal power, across the board, in favor of minimizing individual disempowerment and suffering. Some supporters of capitalism argue that it serves these goals, while other supporters argue that there is no such thing as society and that market forces are in fact the very definition of fairness – which is sharply at contrast with the goals of socialism. (It is therefore critical to understand what supporters of "capitalism" mean when they use the word.)