The Grammar Translation Method (GTM) is an method of second language instruction based mostly on the translation of passages from the native language into the target language. Along the way, students are explained the grammar rules, etymology, and syntax of the target language in a more linguistic than communicative way.
Proposed by Prof. Karl Plotz (1819-1881), GTM is one of the oldest teaching methods in SLA. The strategies were based on learning the strict rules of grammar of the target language, to compare the syntax of the native and target language, and to extract specific sentences or paragraphs to be translated back and forth. It is basically teaching "about" the target language instead of teaching "the" target language.
Plotz method included paradigms, consistent translation of sentences and mirror comparisons grammatical rules. Rote learning (learning by habit in patterns and orally) and breaking down words into lexemes and morphemes were also part of Plotz's original list of strategies.
Now, let's look at how GTM actually works. Using the native language, the teacher will explain to the students how words are formed, placed and used in the target language. Just imagine being in a class where an English-speaking teacher talks about how French words are formed, what they mean, and how they sound. However, you will not ask to learn specific words, nor will you use that information to communicate in French. What you WILL learn, however, is how to use the Grammar rules of the target language in order to translate it into English, and compare the rules of grammar in English in order to translate the passage back into French.
Other examples of GTM include the use of long lists of spelling words (to be memorized) with their meaning i.e, casa-house, perro-dog, gato-cat, etc, not done within a context nor in a communicative flow.
For some learners, the understanding of word formation and grammar may aid a lot in decoding the target language. In other students it takes the fear away of seeing so many different symbols together; yet, it is safe to say that this way of teaching language will not help anyone interact with French people no matter how "bon" you can decode the words.
To modern practitioners of the educational linguistics field, the idea behind GTM is QUITE hard to understand. This is because modern education requires that language learning and teaching are performed within a communicative and academic approach that enhances the innate ability of the learner to learn a different language system. This is one of the biggest drawbacks of this technique.
In conclusion, the GTM method is good as far as its advocacy for the formal study of the target language. However, the fact that students cannot and do not apply the learned concepts within a communicative setting renders this methodology quite "wanting" of further experimentation and added interventions.