Aspects are an important part of modern astrology. as the planets move in their elongated orbits around the Sun, they form various angular relationships with one another, using the Sun or Earth as the center. these are called aspects. the most popular aspects result from dividing the circle by numbers like 1,2,3,4, resulting in aspects such as the conjunction (0 degrees), opposition (180 degrees), trine (120 degrees), square (90 degrees), and so forth. when two planets form an aspect with one another, their energies and natures are said to combine and work in harmony or discord. for example, when two planets are exactly on opposite sides of the Sun (earth), they are in opposition.
Read more, here.