Your original question:

So, perhaps RL could explain what major policy issues differences [in America politics- among those that claim they are socialists, communists, Marxism, or "progressives"].

Your rephrased question:

So, Mary Anne can you explain any major policy differences between American socialists, communists, Marxism, or progressives?

The first incarnation, which you must admit was devoid of syntax, I took to mean "how have the three groups listed differed in their approach to, or effects on, American policy?" I got this from "what major policy issues differences [in America politics". Without the benefit of a verb, which you did not provide and from which I could have deduced in what tense you wanted as the context of my reply, I took a guess at history. I answered your question, as asked.

Thank you for finally putting the question in a form that doesn't require a deck of tarot cards to interpret it.

So, Mary Anne can you explain any major policy differences between American socialists, communists, Marxism, or progressives?

First, in order to have policy, one has to have political power. The groups that you describe, with the possible exception of progressives, have none. So, instead, I will address their political platforms.

Marxism is a philosophy. It has no real political platform, though self-described Marxists often call for revolution. What comes after that, they are somewhat fuzzy about, because they are pinning everything on a philosophy. Marxists, if they belong to political parties, are liable to belong to parties such as the Marxist-Leninist Workers Party USA, which advocates violent opposition to imperialist policies, which it mostly identifies with the US (as the heir to the British empire, which the 19th century Marxists opposed), or to the utopian Marxist group, the World Socialist Party of the US. Utopian Marxists believe that the entire economic structure of the world must be changed, but have rejected the violence and oppression that accompanies the statist attempts to enforce that change. Hence the word, "utopian". The Socialist Labor Party, which touts itself as "The Home of Marxism-De Leonism!" is the oldest party based on Marxism in the US, founded in 1890. It too would be an attractive home for Marxian purists.

Communists have several political parties in the US, many of which have the word "socialist" in them, so I suppose that's where you've become confused. Many are small parties made up of Trotskyite splinter groups, and one, the Progressive Labor Party (which is, I suppose, where you got confused about "progressives"), is a hardcore Stalinist political party. The Socialist Workers Party started out as Trotskyites but have become more like the Fidel Castro fan club. The Workers World Party is more Maoist. And of course there is the Communist Party USA, which has undergone quite a change in approach since Gus Hall died, but whose aims are pretty much the same as they always have been. Generally, the Communists oppose capitalism and most private property, and most of them either favor revolution over democratic elections as a means of change or oppose democratic elections entirely.

Now for the real socialists. The Democratic Socialists of America and the Social Democrats USA are both pro-democracy socialist parties who are members of the Socialist International. If you want to know what that is, suffice it to say that Tony Blair's Labour Party is also a member. The SDU is more centrist and outspokenly anti-Communist than the DSA, which is not to say that the DSA is pro-Communist. Neither has fielded candidates for national office, preferring to use their party status for educational, grass-roots activities. Socialists generally want to provide a large and sturdy safety net for people, including health care, education and retirement. You will also find anti-death penalty activities, support for an end to violence against women, and forgiveness of Third World debt on their list of things to do. They routinely champion fair democratic elections as the most appropriate source of governmental authority.

And now for the Progressives. These are not the people in the Stalinist party described above. In fact, the people likely to call themselves "Progressives" are likely to be very different from each other, espousing favorite liberal causes and causes of various "out" groups. The European Tribune described "Progressives" well:

"Progressive" is an American political term used loosely to identify militants of the left, favorable to sound and fair economic and social policies, equality of chances, and the defense of personal rights.

Of course, you will not consider Progressive economic and social policies sound or fair, because you're a conservative. Bear in mind that the European Tribune identifies itself as left of center.
Progressives are probably more concerned with gay rights than socialists, just as an example. Socialists are more likely to be on the side of the illegal immigrants' worker rights in the current flap, but the Progressives will favor bilingual education for the children of such immigrants. It's that kind of shading. You will find a lot of Progressives in the Democratic Party. They are the noisy ones on the porch, conducting the seance to contact the spirit of FDR. If they get Eleanor, they will be just as happy.

In my travels, I found a Green Nazi Party. Hoo, boy. Wanna look?