Seems simple, right? Poets write poetry, storytellers tell stories, and bards … what do bards do?
In the early Middle Ages, they probably did both of these and more. The Welsh bards, for example, were primarily professional praise poets (the modern Welsh word bardd still means “poet”), but they were also specialists in genealogy, history, mythology, and other traditional “lore” – areas with no well-defined boundaries in those days! In the Mabinogion, Gwydion the magician, while disguised as a bard, entertains Pryderi’s court with “pleasant tales and storytelling,” and the text also says that “he, Gwydion, was the best teller of tales in the world.” Other sources suggest that the bard might serve as a magician or a prophet as well, and the early poets Taliesin and Aneirin were later represented as both (Myrddin / Merlin is a special case, and a different post).
The Welsh laws have references to three grades of bard, and we can assume that the required qualifications increased as you went up. In the Mabinogion example, Pryderi first asked that one of the “young men” traveling with Gwydion, presumed to be his students, should tell a tale, but Gwydion demurred for his own purposes, explaining that it was their custom, the first night after arriving at a court, for the “chief bard” to perform. The Irish situation was considerably more complicated, with a two-fold division into filid (poets) and bardagh (bards), and seven (or more) grades defined in each group. The chief distinctions between the two were that the poet’s position was hereditary and his status due to extensive training as well as ability, while the lower-ranking bard’s position was due to natural ability only.
All clear, then? For those wanting to go farther into the subject, I recommend Law of Hywel Dda, by Dafydd Jenkins, and A Guide to Early Irish Law, by Fergus Kelly. (See also the sidebar link to my Amazon lists, specifically the Early Welsh Bards list.)