The basics: A retrograde occurs when a planet appears to be moving backwards from our vantage point here on Earth. It isn't actually moving backwards, it just appears that way. Mercury turns retrograde most often because it's nearest to us, about 3-4 times per year, and lasts for several weeks. When a planet retrogrades, what that planet rules tends to be thrown off, along with what the sign it's retrograding in rules.
With Mercury retrograde, we experience a shadow and a storm. The shadow period starts when Mercury first hits the point in the zodiac it will turn direct at, and ends when it last hits the point in the zodiac it will turn retrograde at. Most people don't feel the effects of Mercury retrograde until it enters the storm period though.
The storm period of Mercury retrograde is when Mercury is moving at its slowest, less than 40 minutes per day. Mercury is a fast-moving planet and hates it when he's moving slowly. There's usually two storm periods, one around the time Mercury turns retrograde, and one around when it turns direct.
The shadow of this Mercury retrograde begins January 22nd and ends March 20th. The exact retrograde begins February 6th at 4:43PM ET and ends February 28th at 9AM ET. The first storm period is February 3rd - 11th, and the second storm period is February 23rd - March 8th.
The retrograde train continues! Venus turned retrograde in mid-December, and just as she turns direct, Mercury begins. As soon as Mercury retrograde is over, Mars begins, and shortly after Mars is over, we'll have another Mercury retrograde. We don't get a break until July, so be ready!
This retrograde begins in water sign Pisces, and we'll feel the impact of it pretty strong in Pisces since the turn retrograde will be conjunct (in the same spot as) transit (moving) Neptune, which is the ruler of Pisces and is also in Pisces.
We could see problems with addiction, perhaps someone prominent going into rehab; problems with water, like pollution or an oil spill; or problems with confined institutions, like a prison breakout or something with hospitals.
Pisces also rules spirituality, and we could see a scandal with a spiritualist, news about the history of a religion, or a religious issue.
Endings are also Pisces domain, getting rid of what we no longer need, as well as the past and our karma. We could have a hard time letting go, moving on, have past issues come back, karma bites us, and we feel the weight of our baggage.
Our dreams could be more vivid and have more to say to us, though our intuitions may be on the fritz. We can feel strongly that we're sensing the right thing, but it turns out that the opposite of what we thought was the right thing.
Mercury retrogrades backward into air sign Aquarius on February 12th at 10:30PM ET. In Aquarius, we could see problems with rebellions and causes. People don't want to be controlled, and they certainly don't want to be forced into anything against their will.
We rail against conventions and social norms, but struggle to break outside of our comfort zones and be true to who we really are. It just becomes rebellion for the sake of rebellion and not for a point. Governments may try to squash any uprisings that come quickly and with brute force.
We question our dreams, fear what the future holds for us, and can give in to group thinking too easily. Also, watch for any troubles with technology or the stock market.
The turn retrograde begins at 3 degrees Pisces, so those of you Pisceans born February 21st - 23rd may feel the turn retrograde most. The turn direct occurs at 18 degrees Aquarius, so those of you Aquarians born February 6th - 8th may feel the turn direct most.
For the rest of you, check the house(s) Mercury will be retrograding in of your own natal chart, and any aspects Mercury will make during its retrograde to your natal planets, to fully understand how it will impact you personally.
If you have Mercury retrograde in your natal or progressed chart, you may not feel the worst of this. Those individuals generally don't experience the difficulties of the retrograde because it feels more natural to them. Of course, chances are everyone else around them doesn't have the same aspect, so they can still experience difficulties brought on by others.
Suggested Reading: The Mercury, Venus and Mars Retrogrades page for general interpretations