No worries!

Essentially, Social Democracy is a Capitalist system with larger social safety nets, usually as a concession to prevent revolution. Marxist criticism of Social Democracy is that the Nordics, for example, fund their safety nets via brutal exploitation of the global south, and see sliding worker protections and eroding safety nets, because the bourgeoisie is still in control.

Socialism is, generally, a transitional state to Communism. Socialism is categorized by public ownership and central planning of industry as the primary mechanism of the economy (in Marxist terms, there are other forms of socialized production that aren’t marxist, like cooperatives). Examples of Actually Existing Socialist (AES) states include the PRC, USSR (pre-dissolution, obviously), Cuba, Vietnam, Laos, etc.