Edith Wharton, the American writer, said: "art is freedom of the spirit", although if you are looking for a less poetical approach, you might prefer a dictionary: "the class of objects subject to aesthetic criteria."
That means everything from paintings to comics, modern dance to ancient cave images. Art can be made out of precious metal, out of found objects, or out of thin air.
Bloggers writing about art might be asking what their taste says about them, trying to sell their own work, setting out their plans, or reviewing what they've seen. Or they might be making recording the moment on their site.