rob117

The Hebrew prophets were apparently drawing on a long line of Near Eastern prophetic tradition that goes back at least to the Amarna period in the 14th century BC. When the Hebrew culture began to take shape towards the end of the 13th and into 12th centuries BC, they inherited these traditions from the broader Middle Eastern cultural background that their culture was derived from. The prophet in the biblical sense is someone whose purpose is to essentially rebuke people for religious, political, or ethical offenses. Near Eastern prophets were not like Greek oracles. In the Greek tradition, the prophecy of the oracle was unconditional- it would happen, no matter how hard you tried to stop it. In the Iron Age Semitic world, prophets would make predictions of doom, but there would usually be some condition- "But if you repent of these offenses, and cease to do them, these things shall not occur."

Because of their tendency to be overtly negative, opinionated, and iconoclastic, prophets were sometimes (but not always, depending on their connections) shunned by the formal religious establishment (priests), the government and royalty, and by the common people, who in all probability just wanted to be left alone and feed their families. This image is reflected in books like Jeremiah, Hosea, and Amos. Isaiah was apparently on the royal family's good side, so the parts of "his" book that can be plausibly attributed to him (probably less than a third of it) do not reflect this image as strongly.

We have some sparse records of this type of prophet from other Near Eastern cultures, usually from times of crisis. During the height of Assyrian power, there are records of Assyrian prophets rebuking the emperor; as well as of prophets from the cultures that the Assyrians were dominating. The Hebrews were just one of these cultures, so the literature of the Assyrian-era Hebrew prophets (First-Isaiah, Hosea, Amos, and Nahum) is apparently a small sampling of a wider phenomenon of this time.

Basically, prophets were doomsayers, and their popularity suffered because of this. They also did do some weird things "at the command of God/s", such as strip naked, starve themselves, or eat human excrement, that would likely put other people off.

The current concept of the prophet as a seer of the inevitable future- basically, of an oracle in the Greek tradition- comes from the New Testament apocalyptic writings (i.e. Revelation, parts of the gospels, and some of the epistles), as well as late apocalyptic Jewish literature like Daniel. This type of writing was heavily influenced by both Greek and Persian imagery and theology, which is why the Near Eastern prophetic tradition and the Western oracular tradition are often confused and conflated with each other.

So yes, prophecy in the Old Testament sense is basically synonymous with rant.