There are too many websites which provide a handful of conversion tools for both audio and video formats. There are desktop applications too. But many of them are not at all cheap. Some are free, while the others are not. Some free downloads installs malicious apps in the system, without your knowledge or consent.
Here comes the importance of open source applications. Many of which are absolutely free, and does not install any crap software or toolbar without the consent of user. HandBrake is such a Free, open source app with a valid GPL license.
Handbrake is a multiplatform, multithreaded video transcoder, which is available for Mac OS X, Linux and Windows. It can convert from many common multimedia file formats, including unprotected DVD or BluRay sources to a handful of modern output file formats.
[advt]It can encode videos into several popular formats like H.264, Mpeg-4, Mpeg-2, Theora etc. Also it supports MP4 and MKV file formats too. The app encodes audio into formats such as AAC, MP3, FLAC, AC3, DTS, DTSHD etc. The features does not end there, it also support different subtitle formats like SRT, SSA, VOBSub, Closed Captions etc.
Handbrake is not just an encoder/ converter tool, but it also features some editing tools such as DeInterlace, DeTelecine, Deblocking, Cropping and Scaling filters and Presets for common use-cases. HandBrake is a tool for converting video from nearly any format to a selection of modern, widely supported codecs.