These are a commonly used chart particularly in the United States and unlike a simple line chart they are constructed using data of the high, low and close of a specified time/trading period. The trading time period of the bars can be built using minutes, hours, a day, weeks, months, or a year.
They are useful for identifying patterns especially for support and resistance and unlike line charts they give you an idea of how much the market has moved during a given day which is identified by the length of the bar.
The picture to the right demonstrates how a bar is constructed.
The picture below shows how the chart looks when you put bars together.