I apologize if this is a very elementary question, but I'm having trouble finding an answer to it. As we all know, there are many kinds of marriages which one can generally term "halachically disallowed" (incestuous, Jew and non-Jew, mamzer and non-mamzer, kohen and convert, kohen gadol and widow, etc.).

I'd like to know which of these are invalid (in the sense of halachic impossibility: any attempt by the people in the relevant categories to get married is just void ab initio and has no legal force) and which are forbidden (in the sense that a marriage can in fact occur, assuming other ordinary conditions are met, but it will be an aveira and those involved will be duty bound to divorce). The distinction is often not really made clear in secondary sources.

One example I do know about involves polygyny: excluding Yemenites, after the takana of Rabbeinu Gershom, it's forbidden for a married man to marry, but if he does, it's still a valid marriage and requires a get for its dissolution. But what about, e.g., the marriage of a kohen and a convert?