These terms keep changing or at least they are applied to different things. In the case of Trump, populism is the word. And it describes a politician who says what the people want to hear. And here is the new part. He actually tries to enact it, when he gets into office.
Socialism, seems to be a collective of government workers and press and politicians who are trying to stay in power by giving gifts of government to the lower two thirds in hope of winning elections and keep the employment with pension plans and good wages for the government workers.
The reality is that socialism tries to collect the spoils of law making and taxes and spread it, in exchange for votes to a majority of the people.
Populism is different. Its not really that self serving. The populist gets elected by listening to the people and parroting back what he hears. Then if he governs the way that the people requested, he gets to stay in office. If not, a new politician is elected.
Populism is very self-serving.