When The Paint Is Not Oil Based. example: Latex Paint!
Turpentine has many uses. Primarily it is used to clean out paint brushes. It is frequently used as a solvent and also as a paint thinner. It can be used to clean up grease as well.
Turpentine is a natural product used in combination with oils to thin oil paints and "straight up" to clean brushes etc. Paint thinner is not generally as clean a product and I would only use it as a cleaner however I know some people that do use it for thinning paint (commercial oil base house paint) but not for fine art applications.
Oil based paint is not water soluble and so needs paint thinner to clean the brushes etc.
When you've been using latex paint. Then clean them only in water.
You can clean it in hot water with a lot of soap. I do that anyway, after using thinner and it leaves my brushes soft and smooth.
To make the paint easy to spread on surfaces
Dip a rag in some turpentine or lacquer thinner to clean them up. Don't get any on the paint, it will take it off.
Turpentine.
Here are a couple of sites that can explain the "Why's & How-to's":Turpentine is only used to break down the paint on thebrush prior to washing with a suitable soap such as an ecological dishwashingliquid.* http://www.greenspec.co.uk/productpdf/volvoxturps1.pdf* http://painting.about.com/cs/paintingknowhow/ht/Howto_Brushcare.htm* http://emptyeasel.com/2007/01/26/how-to-clean-your-paint-brushes-after-oil-painting/
Paint thinner? There are two basic paint thinners on the market. The old timey one is turpentine. The process of making it is to extract the resin from pine trees then distill the resin to extract the turpentine from it. Turpentine is expensive, so the modern paint thinner is mineral spirits, which is made out of hydrocarbons and hexane.
Safflower oil is used to dilute oil paint and is the base for many oil paints, but is not a solvent. A solvent dissolves the paint completely. Adding more oil to oil paint will only thin the color out into the solution. To dissolve oil paint use a odorless thinner available through art supply stores or turpentine (preferably odorless, a lot of people have a bad reaction to turpentine). Safflower oil will work if you want to thin the paint but will not clean your brushes.
Paint thinner is a product that is similar to mineral spirits. Turpentine is another similar product. All three work to remove paint and help clean up after painting.