Though there are definitely differences in terms of academic aims, conventions, epistemology, etc., I think the best way to understand the difference is as a methodological one. Cultural anthropologists do field work to observe and participate in things that are currently happening (their understanding of which is nonetheless informed by history!) whereas historians are more likely to work primarily with different kinds of documents and written material ("the historical record") to elucidate and interpret past events. Anthropologists and historians often have similar questions about the world, but they tend to go about answering them in different ways (that nonetheless often overlap!)

I don't know as much about archaeology as I do about cultural anth (which I studied in college) but I think one of the big differences is the focus on physical evidence and excavation in archaeology as compared to history, though surely someone else here is more qualified to speak on that than I am.

edit: it's probably easiest to understand them not as two clearly distinguished fields perfectly thought out in relationship to each other so as not to be redundant, but as two different academic traditions, institutionalized in different ways and built around different methods, that have evolved in parallel but nonetheless both have the "human world" as an object of study and often ask similar questions about it :)

Speaking as a moderator: most of the time I refer people to r/AskAnthropology, it is because the question concerns prehistory (the era before written records), the state of modern society, or sweeping transhistorical questions ("why do humans settled into monogamous relationships?" why do so many cultures have marriage?" "what is it about music, seriously?" etc).

History and anthro as academic fields have separate methodologies, although they draw from each other frequently. But on AH, which has quite a few anthropologists among its regulars, a referral to AskAnthro is generally a question of topic coverage. :)