The three grades of oil paints are as follows: artist, professional, and student. The artist-grade paints are categorised as fine, extra‑fine, and super‑fine. The professional-grade paints are sometimes further described as fine.
Artist-grade paints are made without regard to cost. No expense is spared. They are made with the purest and highest quality of raw materials and ingredients to showcase rich and intense colours. Artist-grade paints will contain a mix of traditional and modern pigments that are lightfast. There will be a high pigment loading and little or no fillers. The colour palette will usually have single pigment paints that exceed half of the range.
As for the professional-grade paint, cost plays a role but not at the expense of quality. The professional-grade paints are manufactured to be comparable with artist-grade paints but minus the high price. The pigment loading is usually lower than the artist-grade. The colour palette will usually contain fewer single pigments (usually about one-third of the range) compared to the artist-grade paints and some of the rare and exotic pigments may not be included in the range; however, the pigments used to make the paints will meet the requirements of quality and lightfastness.
Student-grade paints are made in the most economical fashion using the cheapest raw materials and ingredients, as the cost is the primary consideration. The student-grade paints have an average pigment concentration, questionable lightfastness, and are loaded with fillers and other ingredients to create volume. Manufacturers also use mixtures of cheaper pigments to match the colour appearance of more expensive pigments or replace traditional pigments with modern substitutes. There are usually no single pigment colours. Most student-grade paints comprise a mixture of two or three pigments. Often, student grade paint doesn't behave in the same way as the artist or professional-grade paints. That means a student may have to discard some of the knowledge gained during mixing when they upgrade to the artist or professional-grade paints.
(Note: The above information applies to acrylics and watercolours with relevant adaptation.)