In a kitchen, the word “wash” has two senses. One is that of a cooking sense that means to apply something like a light glaze; the other is that of actually cleaning.
In the cooking sense, a wash can be mixed up from an egg, egg and milk, milk, milk and water, etc. A Wash can be used for several things:
- to give a surface a pleasing colour and shine;
- to seal a surface against moisture;
- to help one thing stick to another (such as a fancy cut-out decoration to stick to the top of a pie crust, or seeds to a loaf of bread);
- to dip a food item in before dredging it.
See Egg Wash for a more detailed discussion of this sense of the word.
In a cleaning sense, Wash is a technique that is done as part of food preparation. You use a liquid (usually water) to remove dirt and or (some) germs. Not all home Washing processes can be counted on to kill micro-organisms. To aid in this, a food-grade cleaning agent may or may not be added to the water used.
When washing leafy vegetables, it is easier to wash the leaves whole rather than after chopping. Rub dirt spots on them with your fingers. Fruit and veg should be washed even if you are going to cut and peel them, because your knife may drag any pathogens on the surface into the flesh of the fruit or veg.
Fragile produce can be sprayed with water rather than soaked or scrubbed.
Washing with water won’t remove protective wax coatings that shippers have put on some fruits and vegetables. You can buy special cleansing agents that will do that, though they are somewhat expensive, but they are flavourless, odourless and designed not to leave any residue. Many that you purchase are made largely of vinegar.
As a substitute, you can use:
- baking soda — shake a small amount on, rub and rinse (don’t use on mushrooms);
- diluted vinegar (you can soak fruit and veg in it, but not porous veg such as mushrooms) for 5 to 10 minutes, then rinse. Soak the veg in the sink filled with water and vinegar in a colander to make them easier to collect out of there after soaking;
- Some suggest 1 cup (8 oz / 250 ml) of water with 1 tablespoon lemon juice and 2 tablespoons of baking soda;
- or, in a decidedly more old-fashioned touch, some say just a scrub brush dedicated to produce on veg that can stand up to a bit of scrubbing.
That being said, research done in 2004 by the Food Science and Human Nutrition department at the University of Maine found that, compared with 3 popular brands of commercially produced cleaning Washes, distilled water was as effective in reducing residual pesticide levels, and more effective in removing microbes. 
They consequently recommend:
- soaking fruit and veg in distilled water for up to 2 minutes;
- use in combination with a produce brush for items with thick skin;
- putting fragile produce in a colander and spraying it with distilled water;
- if you buy fruit, etc, to eat on the run during the day, carrying a small spray bottle filled with distilled water to wash it with.
Large-scale commercial egg producers usually wash eggs before packing them for shipping, but some people like to wash their eggs anyway before cracking them as producers have no control over the eggs once they leave their facilities.
 (Kristi Crowe, Alfred Bushway, and Mahmoud El-Begearmi. Best Ways to Wash Fruits and Vegetables. University of Maine Department of Food Science and Human Nutrition, and Cooperative Extension. Bulletin #4336. September 2004. Retrieved on-line from: http://www.umext.maine.edu/onlinepubs/htmpubs/4336.htm June 2006.)