Key Difference:  A Gantt chart is a tool used for project management. It is used to represent the timing of various tasks that are required to complete a project. A milestone chart is used to depict key events along a timescale graphically. Gantt chart may also be used to mark key events known as milestones and therefore in this context, the Gantt chart is also referred as a milestone chart.

Gantt chart and Milestone chart are mainly used in project management. Most of the times, Gantt charts and Milestone charts are often supposed to be the same. However, a milestone chart just depicts one added feature in the basic Gantt chart. This feature is regarding the depiction of key events. Therefore, a Gantt chart can be known as a milestone chart too. Let us explain how these charts work for project scheduling.

Charts are used for pictorial representation and thus are widely used for various purposes. Charts are used to project the relevant information in an effective manner. There are numerous kinds of charts that are used and every chart has its own features and uses. One such chart is known as Gantt chart. Gantt charts are used to project the relation between the task and the time associated with it. It was developed by Henry Gantt in the year 1910. It is mainly used in the field of project management.

Project management deals with scheduling of various tasks and tracking them for the correct implementation. Thus, it is very effective in depicting the work breakdown structure of a project.  These charts help in keeping an eye on the various activities  and phases of a project.  

Gantt charts have two main components-

1. A horizontal axis- it is constructed to represent the total time span of a period. This line is broken into increments to depict days, weeks or months. This timeline illustrates when each activity starts, how long it continues, and when it ends.

2. A vertical axis- it is constructed to represent all the tasks related to the project. For example software training may require tasks like theory lectures and hands-on training.

Now, the most important element that is used to synchronize the task and the time span is the horizontal bar. It may be of varying length as some tasks may take shorter time than the other. As the project progresses, one can use secondary bars, arrowheads or darkened bars to reflect the completed task, or sections of completed task.

In the image of simple Gantt chart, one can easily see that the tasks are indicated on the vertical line and time is indicated on the horizontal line. The time increments are based on date. The horizontal bars are depicting the relation between the date and the task and thus providing the information that on which date the task started and when it finished off.

However, a very big constraint with the Gantt chart is that it cannot be used to depict the dependencies of task on one another. For example if a task does not get completed on scheduled time then what will be the impact of this task on other tasks. Therefore, some other types of charts are also used to depict additional information. Still, because of its simplicity and convenience Gantt charts are used and now various softwares are also available that makes the task easier.

There might be some specific events that are of great importance, but may or may not be related to tasks. These can be marked on the Gantt chart by using a special symbol. The special symbol is often and inverted triangle. Thus, one may also be aware of the milestone event and keep a track on that too. In this particular context, the Gantt chart is also referred as a milestone chart. 

Image Courtesy: examplesof.com, kidasa.com