inflation is a rise in the general level of prices, as measured against some baseline of purchasing power.
Most inflation can be divided into two broad areas: quality theories of inflation, and quantity theories of inflation. Many theories of inflation combine the two. The quality theory of inflation rests on the expectation of a buyer accepting currency to be able to exchange that currency at a later time for goods that are desirable as a buyer. The quantity theory of inflation rests on the equation of the money supply, its velocity, and exchanges. but this two theory have to be combine together.
and the most mainstream economists which is keynesian and There are three major types of inflation
demand pull inflation which is cause high demand for goods and low unemployment.
cost push inflation which is decreasing in supply like event such as a sudden decrease in the supply of oil, or the oil price increasing rapidly
Built-in inflation often linked to the wage spiral because it involves workers trying to keep their wages up with prices and then employers paying higher wage to consumers as higher prices as part of a vicious circle, like what was happened in argentina during during 1990
actually there was lost of theory for explaining reasons for inflation, but the mainstream theory is says above