The dynamically updated chart above uses delayed quotes from Google Finance. It details the interpolation / extrapolation process that computes the 30 day VIXMO from two close-in months of SPX options (VINMO and VIFMO). This calculation is the legacy version of the VIX that was used from 23-Sept-2003 through 5-Oct-2014. On October 6th, 2014 the CBOE modified the calculation to start using SPX weekly options when appropriate—reducing the amount of interpolation/extrapolation required. For more information on that process see Calculating the VIXMO—the Easy Part. VXST is the CBOE’s 9-day version of the VIX, and VXV is the CBOE’s 93 day version.
On 1-Oct-2013 the CBOE started providing the VXST index, which gives a 9-day estimate of volatility. It uses the standard VIX methodology, but uses soon-to-expire SPX options (SPXPM, and SPX). In the chart / data above I show the 20 minute delayed VXST.
There are two somewhat parallel markets associated with general USA market volatility: the S&P 500 (SPX) options market and the VIX Futures market. SPX option prices are used to calculate the CBOE’s family of volatility indexes, with the VIX® being the flagship. VIX futures are priced directly in expected volatility for contracts expiring up to 9 months out. The nearest VIX Future synchronizes with the VIX once a month—on its expiration date.