To understand the mechanism that moves the price up or down we have to learn the interplay between the Depth of Market on one side and Market Orders on the other hand. This interplay is the Order Flow. We used SierraChart Trading Platform for the illustration.
The Current Price is the last price in which a trade took place. This last trade could have taken place either at the Best Bid Price or at the Best Ask Price. Those two terms will make sense in a minute once we understand what the Depth of Market is.
The Depth of Market is the total Size of limit buy or sell orders that are placed at each price. It is often called Liquidity, Limit Orders, Passive Orders or The Book. Take a look at figure #1.