Today, oil paints come pre-mixed in tubes like acrylics and some watercolors. Colors that used to be very expensive to use because the mineral was rare and expensive, like blue, can now be man-made.
Oil paints are glossy and can be transparent (allowing light to pass through) or opaque (blocking light from passing through) depending on the color you choose. The transparent colors seem to glow while the opaque colors are richer. Oil paints colors are very bright, the brightest of any type of paint.
They take much longer to dry than water colors or acrylics. This is useful because it allows the artist to take paint off the canvas easily using turpentine and a rag. Because oil paints dry through a chemical reaction, they continue to dry (and to change) for years after the paint is dry to the touch. In fact, conservators of art (whose job it is to keep art looking the way the artist intended) consider an oil painting to be in the process of drying for up to 80 years! Click on Vermeer’s Girl with the Pearl Earring and you will see cracking across the girl’s face. Next time you visit a museum look for cracks in the paint; you’ll see them everywhere.
Oil paints are different from acrylics and watercolors in that they form a hard shell on the canvas. This shell can crack when the painting is moved and you’ll see fine lines forming through the painting. From the 1500s to the 1800s, it was popular to paint with oil on panels of wood. The wood would sometimes warp though, and this created even more cracking in the paint. To avoid cracking, the paint should get oilier with each layer. This was probably very easy for artists who made their own paints but would be quite difficult for someone today just starting out as a painter.
Oil paints are also harder to clean and will stain if you spill them.