Poster art? Illustrator?

What is 'poster art'? Is it a means of classification of the worthiness of a work of art? On AskArt they have a category of artist called 'Illustrator'. What is the definition of Illustrator? I had originally thought that this was the skill of making exploded drawings of complex objects or machines for the sole purpose of 'illustration', but the artists shown do not seem to be very much different than any other.

Have you ever visited one of those poster sales outlets in a mall? Poster art is what appears on posters - designed for mass sale to mostly adolescent audiences, oftentimes the art that one finds on album covers, etc. and bedroom walls.

Mechanical drawing and drafting are not the same as illustration, and illustration is not the same as fine art, but where one draws the line between them is problematic and open to debate - endless debate. For example: is an architect's drawing of a proposed building complex art, mechanical drawing, or illustration - OR is it all three?

As the best example, people usually refer to Norman Rockwell and his works that were done primarily to 'illustrate' the cover of a magazine (The Saturday Evening Post, primarily). By definition, an illustrator produces art work that illustrates a story - in the case of children's books, as one example, the illustrations are usually accompanied by text. In Rockwell's case, his art told the story - people got the message without further reference to text.