The S Curve is a well known project management tool and it consists in "a display of cumulative costs, labour hours or other quantities plotted against time".The name derives from the S-like shape of the curve, flatter at the beginning and end and steeper in the middle, because this is the way most of the projects look like.

The S curve can be considered as an indicator and it's used for many applications related to project management such as: target, baseline, cost, time etc. That's why there is a variety of S Curves such as:
-Cost versus Time S Curve;(appropriate for projects that contain labour and non-labour tasks).
-Target S Curve;(This S Curve reflects the ideal progress of the project if all tasks are completed as currently scheduled)
-Value and Percentage S Curves;(Percentage S Curves are useful for calculating the project's actual percentage complete)
-Actual S Curve;(This S Curve reflects the actual progress of the project to date)

In order to be able to generate a S Curve, A Baseline and Production Schedule are necessary because they contain important information for each task: -the Baseline - contains information about Actual Start date and finish date.
-The Baseline - can also contain information about Man Hours and costs.
-The Production Schedule contains information about the actual percentage complete.

There are some software scheduling packages that generate automatically S Curves. For example MS Project does not have this possibility so a third party software application is needed to process the Baseline and Production Schedule data and generate the needed S Curve.( for example S Curve Generator that integrates with MS EXcel to generate S Curves).

Sample project s Curve