Water quality is the physical, chemical and biological characterisitics of water. The Clean Water Act requires each jurisidiction to submit biennial reports on water quality in their respective area. This Act is the primary federal law governing water pollution. Maximum contaminant levels are standards set by the EPA for drinking water quality. It is expressed as a concentration in milligrams or micrograms in a liter of water. It is determined by how much of the contaminant can be present, with no ill effects on health. Pollutants include point sources from a stationary, identifiable source such as a pipe or a sewage treatment plant. It includes storm sewers, construction sites, etc. Non-point sources include pollutants that contaminate the water from various sources that accumulate--example, leaching of nitrogen compounds from fertilizing farmlands. The runoff from several farms eventually makes its way to the river or ocean in the area. Pollutants can include organic and inorganic substances. Organic pollutants include: detergents, food wastes, insecticides, herbicides, fungicides, fossil fuels, VOC's, solvents, etc. For the inorganic ones: sulfur dioxide, ammonia, heavy metals, silt and others.
Water quality criteria pollutants would be a list of all the things that affect the quality of drinking water that would be considered a pollutant. The resource link I have provided below gives a comprehensive list of substances considered to be pollutants by several different standards. These substances range from acrylamides, to benzene, to fluoride, and many other substances. Think about a glass of water, crystal clear, necessary to life on Earth. How much pollution, or trash, would you put in that water and still drink it? That would be the other thing you asked about, maximum contaminant levels. It should be noted that these standards differ among all the different nations. What is considered a pollutant by the European nations may not be for the United States, and vice versa. It should also be understood that many of these substances are soluble in water, so they appear relatively invisible.