I am often asked about the differences between oil and acrylic paints. While every artist has their preference I'm going to try to answer this question as best as I am able.
Acrylics work in a very similar way to oils, in that they are semi-opaque and can be applied in thin layers or thick impasto. At a very basic level, there are three main differences between oils and acrylics...
Acrylics dry very quickly, so you can paint over areas that were wet only minutes ago. With oils it's harder to apply more paint to wet areas without smudging all the colors together. Direct painting in oils involves laying down wet paint next to wet paint, and requires a deft and deliberate hand. However, there are benefits to painting wet on wet, such as the ability to achieve soft edges. A shorter or longer dry time can work for or against you depending on the effects you are trying to achieve.
Oil paints are made of pigment blended with oil (linseed, safflower, walnut, etc.). Acrylic paints are made of pigment blended with acrylic polymer emulsion (a synthetic resin).
To thin oil paint you can add oil or a solvent such as turpentine or mineral spirits. To thin acrylic paint you can add water or an acrylic painting medium. There are countless mediums that can be added to each type of paint that vary the consistency, texture, transparency, and dry time of the paint. Brushes used with acrylic paint can be cleaned with water and a mild soap, and brushes used with oils can be cleaned with oil, mineral spirits, and/or a conditioning soap such as Murphy's Oil Soap or the Masters brush cleaner.
There are other differences too. Acrylics don't require solvents or other painting mediums that some people are sensitive to, so they can be a better choice for anyone with allergies or chemical sensitivities. However it's possible to use oil paints without solvents, and there are solvent free mediums on the market.
Bottom line: They each have their advantages, and I enjoy working with both of them.
Did I leave anything out? What is your favorite medium and why? I'd love to hear your thoughts.