Wikipedia
DirectVobSub (formerly known as VSFilter) is a software add-on for Microsoft Windows (a DirectShow filter) that is able to read external subtitle files and superimposes them on a playing video file.
DirectVobSub/VSFilter were formerly part of a whole application known as VobSub which was also able to extract subtitles from DVD Video and create text-based subtitles, without ripping the DVD to a file first. The last version of VobSub was version 2.23, after which the development of VobSub ceased. VSFilter was a part of the guliverkli project on SourceForge web site. The guliverkli project also includes the ability to extract subtitles from a DVD via the vobsub ripper program. However, development of guliverkli ceased in 2005 with version 2.37. In 2007, it became a part of the Guliverkli2 project, now known as DirectVobSub, starting with version 2.38. But till September, 2012, the last guliverkli2 commitment was in April 10th, 2011. Then, the MPC-HC project took over the sources. Started with 2.40, DirectVobSub is in active development again.
DirectVobSub is also capable of extracting subtitles from a DVD without ripping it to a file first. "Vobsub rippers" are also available for the same effect. Note also the guliverkli project has a related project VSrip.