He was a liberal Democrat.

Though he introduced reforms during difficult times, he was patriotic to America's democratic and capitalist institutions.

America's social safety net was inadequate during the Great Depression so he took action to remedy that.

Socialism is a different thing entirely (though ideologues today are attempting to blur distinctions). In short, socialism is a system where wealth and the means of production are in the hands of the government.

Capitalism is a system where wealth and the means of production are in the hands of the private sector. Like in a Monopoly game, there is need for fair rules for all, but social programs and regulations are not socialism and FDR was not a socialist. He was a Democrat who supported democracy and free enterprise but intended to improve it.