When measuring gases like carbon dioxide, oxygen, or methane, the term concentration is used to describe the amount of gas by volume in the air. The 2 most common units of measurement are parts-per-million, and percent concentration.
Parts-per-million (abbreviated ppm) is the ratio of one gas to another. For example, 1,000ppm of CO2 means that if you could count a million gas molecules, 1,000 of them would be of carbon dioxide and 999,000 molecules would be some other gases.
When you start counting over 10,000ppm, most scientists and manufacturers change from ppm to percent concentration. So instead of describing a 10,000ppm CO2 sensor, we talk about a 1% CO2 sensor instead. (10,000/1,000,000 = 0.01) – the terms are synonymous. In other words, 1ppm = 0.0001% gas.
The advantage of ppm and % concentration is that, under typical operating conditions, temperature and pressure are not important when comparing 2 different sensors specifications.
Keep in mind that while ppm and % concentration tell you the ratio of one gas to another, they don't tell you the weight of the target gas. This is why you will occasionally see the molecular weight (mg/m³, or milligrams per cubic meter) of a gas in a specification. There are easy gas conversion calculators on the web you can use, or you can make a quick, approximate conversion (assuming 77 degrees F and sea level) for CO2 only by multiplying the ppm by 1.8 to get mg/m³, or dividing mg/m³ by 1.8 to get ppm.