The following MPUG article by Kenneth Steiness is a good introduction to the basics of baselining. A baseline is a snapshot of your project at a point in time and you can create up to 11 of them. Baselines provide a benchmark to compare against and also provide variances. The variance helps show how far you were or are off of the original estimate, so you can take corrective actions. This can help avoid delays, cost overruns, resource issues, and much, much more.
Here's the article: http://www.mpug.com/articles/baselining-best-practices/
As you go through the article, under the Maintaining the Baseline section, look at how he uses inactive tasks.