The aspects in the horoscope are formed by the mathematical angle between two planets in the chart. There are many aspects but the primary ones are the Ptolemy aspects: the conjunction, sextile, square, trine, and opposition. Each aspect has an energy which can represent an open door for easy expression of the planets or a narrow passage that inhibits expression.
The conjunction is when two or more planets in the chart are close together, sitting side by side. This aspect merges the energies of those two planets and offers the gift of new beginnings, power, and energy to thrust forward.
The sextile is formed when planets are at a 60 degree angle to each other. This aspect carries a very fortunate energy with it because it creates opportunities to produce something, take a new avenue, to act on a new creative endeavor.
The Square is created when planets are at 90 degrees to each other. Just think of walking around a square. It's not as easy as walking around a circle or a sharp angle. Your view is peripheral rather than face-to-face. So, planets in square create tension, often stress and obstacles to expression.
The Trine is another easy aspect created by an angle of 120 degrees between planets. This offers the greatest ease of expression, harmony in action, and takes little effort to complete the task. Sometimes, though, it can create laziness and opportunities can be missed.
The opposition is an angle of 180 degrees. With it, the ability to see directly in front of you is offered. This creates awareness and with that the ability to clearly see what needs to be reworked with cooperation and compromise.