An easier way to illustrate this is to imagine two cars moving down the highway. The first car (Earth) is moving at a speed of 60 mph. The second car (Mars, for example) is about a half mile ahead of the first car, but is moving at 55 mph. As Earth approaches Mars, and pulls alongside the slower car (planet), it is as if both cars (planets) are standing still, and then the slower car (planet) appears to roll backwards as the first car pulls ahead.
All planets have a retrograde period, though they are at different times and for different intervals, depending on their sidereal cycle. Mercury is the closest to the Sun, and thus has the shortest sidereal cycle (88 days). Mercury has an average of three retrograde cycles per year, each lasting for about 3 weeks. Conversely, Pluto's sidereal year is 90,465 days. Pluto "goes retrograde" once per year as the earth passes it, and stays retrograde for about 5 months at a time!
While it is true that the energy of a given planet is affected during its retrograde period, it is not necessarily a negative occurrence. Since it happens most frequently, allot of attention is given to Mercury Retrograde. Since Mercury rules communication and travel, some people do experience trouble with their travel plans or modes of transport (late trains, car trouble) and modes of communication (computers slow, writer's block), but Mercury Retrograde also gives us a chance to slow down and regroup from the frenzied pace of life. A rule of thumb is this is a good time to do anything beginning with "re-"...Relax, re-examine, re-evaluate, re-connect, etc.
If you want to be more specific, look at what sign the planet is in when it turns retrograde. For instance, if Mercury turns retrograde at 15 degrees Libra, that could indicate a warning to pay closer attention to any decisions made relating to partnerships or that some things may seem out of balance.
Other terms associated with a retrograde period: