What is Grammar?

Grammar is the structural foundation of our ability to express ourselves. The more we are aware of how it works, the more we can monitor the meaning and effectiveness of the way we and others use language. It can help foster precision, detect ambiguity, and exploit the richness of expression available in English. And it can help everyone--not only teachers of English, but teachers of anything, for all teaching is ultimately a matter of getting to grips with meaning.
(David Crystal, "In Word and Deed," TES Teacher, April 30, 2004)

It is necessary to know grammar, and it is better to write grammatically than not, but it is well to remember that grammar is common speech formulated. Usage is the only test.
(William Somerset Maugham, The Summing Up, 1938)

Hear the word glamour and what comes to mind? Celebrities, most likely--limousines and red carpets, swarms of paparazzi and more money than sense. But, odd as it may sound, glamour comes directly from a decidedly less glamorous word--grammar.
During the Middle Ages, grammar was often used to describe learning in general, including the magical, occult practices popularly associated with the scholars of the day. People in Scotland pronounced grammar as "glam-our," and extended the association to mean magical beauty or enchantment.
In the 19th century, the two versions of the word went their separate ways, so that our study of English grammar today may not be quite as glamorous as it used to be.

There are two types of Grammar: Descriptive Grammar and Prescriptive Grammar

Descriptive Grammar
refers to the structure of a language as it is actually used by speakers and writers.

Prescriptive Grammar 
refers to the structure of a language as certain people think it should be used.

Both kinds of grammar are concerned with rules--but in different ways. Specialists in descriptive grammar (called linguists) study the rules or patterns that underlie our use of words, phrases, clauses, and sentences. On the other hand, prescriptive grammarians (such as most editors and teachers) lay out rules about what they believe to be the “correct” or “incorrect” use of language.

The study of grammar all by itself will not necessarily make you a better writer. But by gaining a clearer understanding of how our language works, you should also gain greater control over the way you shape words into sentences and sentences into paragraphs. In short, studying grammar may help you become a more effective writer.