The PSM means the Palestine Solidarity Movement, the name the ISM calls itself inside the US. When over in the West Bank aiding terrorists they call themselves the ISM or International Solidarity Movement.
Several different PLO propaganda groups circulate around the Untied States, mostly on college campuses as a base. The word "Solidarity" connotes some revolutionary groups from past history. American communists have embraced the ISM because as an extension of the communist inspired PFLP (Popular Front for the Liberation of Palestine) terrorist group, the ISM seeks to recruit communist and anarchist radicals on America's campuses to work for the Palestinian cause. The ISM/PSM now also works with Hamas and Islamic Jihad. In fact, Huwaida Arraf, a U.S. leader admitted at Duke University the ISM will work with any terrorist group to destroy Israel.
The PSM consists of groups like Al Awda (which advocates the dismantling Israel by using the euphemism "right of return" for overrunning the Jewish population in Israel with Muslim Arabs). Oddly enough, Al Awda is run by Mazin Qumsiyeh, a Christian Arab.
Why would a Christian Arab promote Islamic hegemony in Israel, especially since Christians are persecuted in the West Bank? Walid Shoebat, a former PLO member explains it this way: Qumsiyeh is a product of Islamic culture having grown up in Islamic schools where destroying the Jews is taught almost as soon as a child can walk.
Other groups in the PSM include SUSTAIN (Stop American Taxpayer Assistance To Israel Now), the General Union of Palestinian Students, Students for Justice in Palestine ("justice" meaning the end of Israel, the Alternative Palestinian Agenda (at University of Wisconsin-Madison, a hub of ISM-PSM activity in the USA). Multiple layers of groups were created in local regions to 1) make their numbers seem greater 2) avoid exposure for liability reasons 3) create regional and local control where applicable.
They are all going to meet at Georgetown this month to train and strategize to destroy Israel.