Gantt charts are a way of keeping track of projects you're doing so that you can manage them more effectively. More than just a list of tasks to work on, Gantt charts provide a visual component to help you see which jobs must be done consecutively or concurrently to help you stay on top of your workload.
Bright Hub has a series of useful articles on using Gantt charts:
There are computer-based tools for creating Gantt charts, such as Microsoft Project for Windows and OmniPlan for Macintosh, but they tend to be expensive. Web-based tools tend to require paid subscriptions, though some offer an option for a limited free account.
10 Free Online Project Management Tools
It definitely packs the most features of any free (and a few for-pay) project management tools on the Web.
In addition to the standard task lists, time tracking, and schedules, you also get features like a discussion area, the ability to upload files related to a project, email notifications, and built-in messaging. You can even keep track of projects using RSS feeds. Not bad for a free tool!