Gantt charts show the start and finish dates of the elements of a project. Gantt charts show the "dependency" between activities. What this means is that some activities can only begin after other activities are finished. Other activities can go on in parallel. Gantt charts can be used to show current schedule status using percent-complete shadings and a vertical "TODAY" line as shown here.
They are now a common charting technique, but Gantt charts were thought revolutionary when first introduced. The charts are also used in information technology to represent data that has been collected.