Vyakarana or grammar is the "mukha" of the Vedapurusa, his mouth. The Tamil word for grammar is "illakanam". Grammar deals with the "laksanas" of a language. "Laksmana(n)" is "llukkumanan" in Tamil. In the same way, "laksana(m)" becomes "illakanam" in that language.
There are a number of works on Sanskrit grammar. The most widely used and important is the one by the great sage Panini. There is a gloss - a vartika- on his "Vyakarna-sutra" by Vararuci. Patanjali has written a bhasya or commentary on Panini's sutras. These three are the chief works on Sanskrit grammar.
There is a difference between grammar and other sastras. In the case of other subjects the original sutras constituting them are esteemed more than their bhasyas. But, in the case of grammar, or Vyakarana, the Vartika is more valued than the sutras and still more valued is the bhasya.
According to one reckoning, there are six sastras. Vyakarana is one of them. Four of the sastras are particularly important :apart from Vyakarana, Tarka(logic), Mimamsa and Vedanta. Vyakarna is also one of the vedic sadanga( six limbs of the vedas ).
"Sucant sutram ", so it is said. (The sutra is just an indication of something, a truth or a principle. ) Every sastra has a bhasya and each such bhasya is known by a particular name. The vyakarana bhasya (of Patanjali) alone is called "Mahabhasya", "the great commentary ".
For a general background, please see here