Both of these terms are units of measurement. Measurements most have units (or dimensions) with them. Both of the terms, ppm and ppb are ratios that show concentration of one substance compared to another usually by weight. The first is parts per one million and the second is parts per one billion. (Many do not realize that the more common symbol “%” is actually designating parts per one hundred. )
When these measurements are associated with solutions, ppm would more accurately be designated as milligrams per liter. (A liter of distilled water weighs one kilogram.) For example, if a water sample is tested and determined to have 10 ppm of Nitrate, this solution has 10 milligrams of Nitrate per liter of water. If a water test shows 50 ppb of Lead, it would more accurately be expressed as 50 micrograms of Lead per liter of water, and so on. (This is quite often the way results will be expressed at a water testing facility.) For simplicity, water standards are generally listed with ppm and ppb designations.
It is very difficult for the typical layman to visualize the terms such as ppm and ppb. It usually helps to compare these terms to common everyday measurements. For example: a part per million is like one inch in 16 miles or one drop in 35 gallons. Extending this concept, one ppb is like one inch in 16,000 miles or one drop in 35,000 gallons. There are now laboratory methods for measuring down to parts per trillion and parts per quadrillion but we won’t go into that