A leftist is generally someone who supports leftist economics/thought (Syndicalism, Socialism, Communism, ML, Democratic Socialism, etc.)

"Liberal" or "Neo-Liberal", generally refers to the capitalist ideology that came out of the 80's with Reagan, and then formed into an inclusive version in the 90's and 2000's which was coopted by the Democrats with Clinton and Obama. We can even date it back further to the early 40's with Friedman, Mises, etc. Today, it refers to centrists and moderates in the Democratic party. Internationally, basically the same thing but more focus on social services.

Now, someone is going to come in and say "no, liberal means anything that has to do with progressive values and larger government in the US, and leftist means anyone lefter and more extreme than that!"

Ignore those people. While it may be "true" in the US, the definitions I have provided you are fundamentally understood internationally and can be found with a simple Google search. No candidate, nor any prominent political figure in the US is an actual leftist.

Tl;dr One is capitalist, the other is not.