The Bible explicitly tells (I Sam. 9:9) that what was called a navi in later times was the same role as what used to be called a roeh in earlier times. The Malbim explains that in earlier times a prophet was called a roeh because the main function of the prophet was to “see” with his Divine Inspiration, and use that to help individuals discover that which was hidden from them (whether in terms of lost items or self-improvement). For example, when Shaul lost his father’s donkeys, he sought out the prophet Samuel to either help him find them or figure out what sin he had committed which led to this loss (I Sam. 9). In essence, a roeh was sought out by those wished to consult with them.
In later generations Jewish society degenerated, and a new type of prophet emerged: A navi was G-d’s messenger sent to rebuke the nation for their sins, and bring them back to the proper path. The navi did not generally speak to the individual, but to the public at large. As opposed to the roeh who was sought out, the navi was a feared character, from whom people tended to run away.
The Vilna Gaon (to Prov. 22:12, Isa. 1:1) explains that the terms navi and roeh/chozeh focus on different aspects of a prophet’s function. The word navi, as we mentioned above, is related to the word niv which refers to the prophet’s mouth.
That word focuses on the prophet’s role in relaying with his mouth G-d’s message.
However, the words roeh and chozeh refer to the prophet “seeing” a certain vision which he is to convey to others. Within this role of the prophet, the Vilna Gaon explains, there are two types of Seers: One is called a roeh,which is the Hebrew word for “he who sees”, and was the term used for the earliest prophets, whose clarity in the visions they saw was quite sharp. A later prophet, by contrast, is called a chozeh, the Aramaic word for “he who sees”. Because Hebrew is considered a more spiritually-attuned language than Aramaic, later prophets are referred to by an Aramaic term for “Seer”. This shows that those prophets were not as clear on the meaning of their visions as the earlier prophets were. For this reason, a later prophet is called a chozeh (and his vision a chazon/machaze), and an earlier prophet, a roeh (and his vision is called a mareh).
SOURCE: What's in a Word?, "Prophets and Visionaries"
