Generally, "pure" pixel art must be created by manual pixel-level editing, without the use of any automatic filters. In the pure form, it is generally said that "each pixel was placed carefully". Purists within the pixel art scene say that pixel artists should only use tools that place individual pixels (usually the pencil tool), and no tools that automatically create shapes for you (like circle, square and line tools). Others say that the line tools and the bucket fill are acceptable, as they do nothing more than speed up the work without impacting how the work looks. The use of automatic filters such as anti-aliasing, on the other hand, is generally considered not valid in "true" pixel art, since the filters will add new pixels automatically, eliminating the careful placement.
Also, a common mistake is to think that any drawing or doodle done using the pencil tool is pixel art. This is not true, since pixel art is categorized by the method of drawing (pixel by pixel), not the results (therefore, special renders and automated filters do not apply either). Such drawings are actually called oekakis.