Bar charts provide traders with four key pieces of information for a given time frame: the opening price during that time frame; the closing price; the high price; and the low price. Bar charts can be applied to all time frames, and hence a single bar can summarize price activity over the past minute or over the past month. Different traders use time frames in various manners, although a good rule of thumb is that the longer the time frame, the more significant it is as it will account for more data -- and hence will be a better reflection of the market's psychology.