A sextile occurs in a chart when two planets are 60 degrees from each other. An orb of six degrees is considered acceptable for this aspect. This is one of the major Aspects, and a soft energy is generally ascribed to it.
Since sextiles, by virtue of their sixty-degree separation, are usually two Signs apart, we tend to see positive Signs sextiling each other. By extension, negative Signs sextile each other as well. For example, it should be expected that Fire and Air Signs sextile each other, and that the same relationship will correspond between Earth and Water Signs. These relationships aid in the sympathetic energies between the planets involved.
Sextiles are about the easy and unencumbered flow of energy between two planets. There is a lack of tension inherent in this aspect which translates to the ability of the individual to accomplish much. Attraction is present, and self-expression is enhanced. It's important to note, however, that while a sextile presents the individual with opportunities, the responsibility is still on them to make things happen. A sextile isn't the answer to everything, but it's certainly a step in the right direction. Again, a sextile is harmonious and engenders cooperation.