Inflation & Deflation
Inflation occurs for two reasons. The first is when the money supply increases which means the money you already own is worth less because it has devalued. The second is when the velocity of money increases which means money is flowing around the system more quickly.
Since 2008 the money supply within the financial sector has increased dramatically along with the velocity due to QE. This has resulted in numerous bubbles being produced with the prices of shares, bonds, commodities and other markets like derivatives and real estate increasing dramatically to heights never seen before. Conversely, in the real economy profits and wages are in decline because very little of this extra money is leaving the financial sector and the velocity has slowed because people and businesses are very concerned about spending money that they don't have. This is a symptom of financial sectors growing to large in proportion to other parts of the economy. Another side effect is that too much trading on the commodities markets results in price increases for everyday goods for people living in the real economy regardless of supply and demand (Price Discovery). Prices now depend on the whims of traders, high frequency trading by computers and the fraudulent practices of those with inside knowledge manipulating the markets.
In many advanced economies outside of the world of high finance deflation is a major concern. One of the main reasons is that since 2008 the velocity of money has sharply declined. Large international businesses and corporations have been investing huge amounts of their profits into buying their own shares on the stock markets. This means even more more money being sucked into the financial sector from other sectors. These businesses are also building massive reserves of money in offshore tax havens rather than investing in innovation and research or their workforce. So there is also a knock on effect of less quality jobs and lower wages. Lower incomes and less jobs means that people don't have so much money to spend which means less demands for goods and services which means less profits for businesses owners and in essence the beginnings of a deflationary spiral.