Fluorescence Correlation Spectroscopy (FCS) is a powerful fluorescence spectroscopy tool with single molecule sensitivity which has found widespread usage in the life sciences. This CDF file aims to provide some basic explanations for FCS and provide some simple applets so that new members in the field can gain a better understanding of fluorescence autocorrelation functions and how their parameters change with the properties of the system. The Wolfram CDF player needs to be installed to view CDF files. The CDF player can be downloaded here.