On a daily chart, each bar represents the high, low and close for a particular day. Weekly charts would have a bar for each week based on Friday's close and the high and low for that week. Bar charts can also be displayed using the open, high, low and close. The only difference is the addition of the open price, which is displayed as a short horizontal line extending to the left of the bar. Whether or not a bar chart includes the open depends on the data available.
Bar charts can be effective for displaying a large amount of data.
The individual bars that make up the bar chart are relatively skinny, which allows users the ability to fit more bars before the chart gets cluttered. If you are not interested in the opening price, bar charts are an ideal method for analyzing the close relative to the high and low. In addition, bar charts that include the open will tend to get cluttered quicker. If you are interested in the opening price, candlestick charts probably offer a better alternative. For a candlestick chart, the open, high, low and close are all required.