In general, Mercury rules thinking and perception (http://www.astrologycom.com/mercret.html), processing and disseminating information and all means of communication, commerce, education and transportation. By extension, Mercury rules people who work in these areas, especially people who work with their minds or their wits: writers and orators, commentators and critics, gossips and spin doctors, teachers, travellers, tricksters and thieves.

Mercury retrograde gives rise to personal misunderstandings; flawed, disrupted, or delayed communications, negotiations and trade; glitches and breakdowns with phones, computers, cars, buses, and trains. And all of these problems usually arise because some crucial piece of information, or component, has gone astray, or awry.

It is therefore not wise to make important decisions while Mercury is retrograde, since it is very likely that these decisions will be clouded by misinformation, poor communication and careless thinking. Mercury is all about mental clarity and the power of the mind, so when Mercury is retrograde, these intellectual characteristics tend to be less acute than usual, as the critical faculties are dimmed. Make sure you pay attention to the small print!