MKVToolNix is a set of tools to create, alter and inspect Matroska & WebM files under Windows, macOS, Linux and other Unices. It is the de-facto reference implementation of a Matroska multiplexer.
MKVToolNix consists of the following command-line tools:
mkvmergeis a tool to create Matroska & WebM files from other formats.
mkvinfoallows one to get information about the tracks in Matroska & WebM files.
mkvextractcan extract tracks from Matroska & WebM files to other formats.
mkvpropeditcan edit properties such as header and chapter information or attachments without remuxing.
On top of them sits MKVToolNix GUI, an easy-to-use program making the functionality of those command-line tools available as a GUI.
The program can be downloaded from its home page