First of all, every chart has two nodes-- a north and south node. If you only have one listed in your chart, it is probably the north node. These nodes may be calculated as per the true location of the node on the orbit (accounting for gravitational effects of the Sun, for example) or the mean location. These are usually very close, though the mean location is always retrograde and the true location can occasionally go direct for a couple days.
The nodes are simply the places where the Moon's orbit crosses the ecliptic (sun's apparent path through the stars). The north node is where it crosses while moving north of the ecliptic, and the south node is where it crosses while moving south. Orbits being great circles, it is not possible to have just one node and they are *always* 180 degrees apart.
Now, when you have the sun and moon in an exact conjunction on a node (either one) you have a solar eclipse. If you have the sun on one and the moon on the other, you have a lunar eclipse. Hence due to the old myths of the dragon Typhon eating the sun, these have been called the Head of the Dragon and the Tail of the Dragon respectively (Norse myth suggests that they are the wolves Skoll and Hati).
In general, the North Node is seen as representing a directing force while the south node represents either a hoarding or prosperous force but one of little action. In Horary Astrology, the South Node is malific while the north node is benific, though in natal astrology one could make a case that the roles are reversed. In horary astrology any planet on a nodal degree (of any sign) is considered somewhat afflicted. I.e. if the node is at, say, 19Ari42 and a horary chart has Jupiter at 19Vir20, then Jupiter is somewhat afflicted. The reason has to do with something called prefection which is an old technique somewhat similar to (but very different from) projection. In prefection, you advance everything in the chart exactly one sign forward every year. This is still done in Vedic astrology and is covered (in Western Astrology) by Bonatus, Maternus, and others.