GRU: What is the Russian military intelligence service that has been connected to the Salisbury attacks?

Alexander Petrov and Ruslan Boshirov (not thought to be their real names) are alleged to have carried out a nerve agent attack in Salisbury in March, which poisoned Sergei and Yulia Skripal.

British officials have linked the two suspects to a Russian organisation called the GRU (or Main Intelligence Directorate).

So what do we know about this secretive organisation?