Avgas (meaning Aviation Gas) is a high-octane fuel used for aircraft and racing cars. The avgas title helps to distinguish the fuel between mogas (motor car gas (used in the typical automobile)). Some aircraft are certified to operate on both mogas and avgas but most small aircraft operate on avgas due to its higher lead 
content. Turbine and diesel engines are designed to use kerosene-based jet fuel. Avgas has a lower volatility than mogas which basically means it does not evaporate as quickly. This is very important for high-altitude use and higher temperatures. The mixtures in use today are the same as when they were first developed in the 1950s and 1960s. The higher octane ratings are achieved by the addition of lead, a highly toxic substance that was phased out for car use in most countries in the 1980s. Avgas is currently available in several grades with differing maximum lead concentrations. Different grades of avgas can be easy to confuse so dyes are added to help distinguish between different grades. For example, 100/130 grade avgas has a green tint and 80/87 octane is tinted red. 100LL (the LL stands for low lead) is tinted blue. If any of these fuels are mixed, the solution will turn colorless to indicate a mixture. Since lead is a rather expensive additive, a minimum amount of it is typically added to the fuel to bring it up to the required octane rating.
Evan Krueger
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