Essentially, a Gantt chart is a bar chart used to illustrate how much time it will take to complete various tasks in a project. For example, here's the simplest form of Gantt chart produced with the Excel spreadsheet.
The worksheet shows, for instance, that Task 1 began on Jan 1, 1991 (OK, it's out of date.) Much of the task had been completed by May 30, 1991 and there was still some part of the task left to be done. It appears that the whole project would have been done by the first week of Nov., 1991.
For a more sophisticated Gantt chart, see the document that shows talks about Primavera. This will lead you to the article that deals with the development of a software project.
The Gantt chart lets a planner get an overview of an entire project. They force the planner to think about all the activities involved and put some time limits on the different tasks. The chart also shows how one task my begin while another one is underway. On the downside, simple Gantt charts do not show which task must be completed before another one is begun. There are PM tools such as Primavera and Project: VISION which will let you incorporate this information if you want to.