The concept of Sect comes to us from Hellenistic times. It is a concept that fell out of use for many years, but is being rediscovered as more and more astrologers are re-examining ancient techniques. Sect can help you determine the qualities of a planet, whether for good or ill and its effect on the chart.
Depending on whether it is a daytime or nighttime chart, sect tells you which of the classical planets (Sun and Moon, and the planets Mercury through Saturn) have the upper hand. It also provides clues as to which planets will be more helpful or more problematic in the chart.
As with any technique, there are nuances. But the basic process for using sect is simple. First, find the sect of the chart. This is easy - where's the Sun? If it is above the horizon (the ascendant/descendant axis), it is a diurnal chart that emphasizes the Sun, Jupiter, and Saturn. If the Sun is below the horizon, it is a nocturnal chart that emphasizes the Moon, Venus, and Mars. Mercury's sect is determined by whether it rises before (oriental) or after (occidental) the Sun.
Next, determine whether or not the planets are in their natural sect. For example, Saturn is a diurnal planet. If it is above the horizon in a diurnal chart, it is matched with its natural sect. This gives Saturn an even extra boost as it delights in its placement. Third, consider whether the planet is in a masculine (diurnal) or feminine (nocturnal) sign.
Planets that are in their natural sect by hemisphere and in a sign which matches their natural sect are in hayz. In Medieval astrology, this was considered a particularly beneficial and powerful condition. If a planet is out of sect by hemisphere, sign or both (a condition Robert Hand calls ex conditione), it will have difficulty expressing positively in the chart.