How aspects are measured
Each aspect is a specific angle along the zodiac. Planets rarely sit at the exact angle, so astrologers allow an orb, a margin of a few degrees. A tighter orb is read as stronger. Orbs are a convention, not a law of nature, so this site states the ones it uses.
Conjunction
0° apart, with an orb of up to 8°
They operate as one voice, so you may not notice where one theme ends and the other begins.
Sextile
60° apart, with an orb of up to 4°
They cooperate when you give them a small, deliberate opening. The talent is real and easy to underuse.
Square
90° apart, with an orb of up to 6°
They pull in different directions. The friction is useful once you stop trying to silence one of them.
Trine
120° apart, with an orb of up to 6°
They recognize each other easily, which can become a talent you forget to practice.
Opposition
180° apart, with an orb of up to 8°
They face each other across the chart. The work is to let both stay in the room rather than living at one pole.
Easy and hard aspects
Trines and sextiles are called easy aspects and squares and oppositions hard ones, but neither label is a verdict. Easy aspects describe talents that can go unused. Hard aspects describe tensions that tend to push a person to grow. A chart with only trines is not a better chart.
Applying and separating
When a faster planet is moving toward the exact angle, the aspect is applying. When it has passed it, the aspect is separating. Applying aspects are traditionally read as themes still gathering strength, and separating ones as themes already familiar.