A belles-lettre is something which fits into the category of literature known as belles-lettres, from the French for “fine letters.” The types of literature which fit into this category are a bit nebulous, and the meaning of “belles-lettres” has shifted over the years. Someone who writes literature which falls into this category may be called a belletrist.
Originally, this term was used to refer to literature with a focus on aesthetics, rather than content. A belles-lettre was something elegantly, beautifully, and stylishly written, with features such as innovative word use, vivid visual images, and general beauty. Poems, novels, essays, and other forms of writing could all fall into this category. Reading such work is supposed to be an aesthetic pleasure, with people enjoying the beauty of the work for its own sake.
In some cases, a belles-lettre is also literally beautiful. Historically, the typography in such works was done with special care to make the work physically appealing as well as beautiful to read. Typographical tricks could be used to add an air of play or whimsy to the piece, and the aesthetics of the piece as a whole were carefully considered. Physical beauty was valued along with the aesthetics of writing, in other words.
One time, the term “belles-lettre” came to be used in reference to any sort of light literature which is short on substance and designed to entertain or enrapture. Along the way, it acquired some pejorative connotations in some circles, with people dismissing belles-lettres as being beneath their interest. College and university programs which focused on studying and producing such works, for example, might be accused of diluting the purpose of the written word. Likewise, authors who focus on this type of writing may be derided by critics.
Belles-lettres certainly have their place. The aesthetic enjoyment of literature is something people have appreciated for centuries, and while more dry works focused on informational content are also valuable, sometimes it is pleasant to be able to enjoy a work simply because it is a pleasing work of art. Much as something like a painting does not have to serve a practical or informational purpose, a belles-lettre can be a way to lighten and enrich the mind in an aesthetic sense. People may also enjoy crafting such works, as the demands on the writer are very different than those for people working on informational, critical, and substantive pieces. The ability to write elegantly and well can actually be very valuable for people who want to convey information or present arguments.