The angles that count
An aspect is a particular angle between two bodies, measured along the ecliptic. The five major ones are the conjunction at 0°, the sextile at 60°, the square at 90°, the trine at 120° and the opposition at 180°. The quincunx at 150° and the semisextile at 30° are minor.
Modern astrology calls the sextile and trine easy and the square and opposition hard. The words describe the feel of the angle, not good or bad luck, and the conjunction is neutral. Aspects to the Ascendant and the Midheaven count as well, which is why they need a birth time.
Lines on the wheel
On the wheel each aspect is a line between two bodies. The closer to exact, the thicker and brighter the line, so the tight aspects stand out and the wide ones recede. Easy aspects are drawn in mint, hard ones in pink, conjunctions in white and minor ones dashed.
An aspect joins two positions. It cares about the degrees between them, not the signs they are in. Two bodies in neighboring signs can be conjunct, and two in the same sign can miss each other.
How close is close enough
Angles are almost never exact, so each aspect has an orb, the margin around it. Genastro counts an 8° orb for conjunctions and oppositions, 7° for trines and squares, 5° for sextiles, 3° for quincunxes and 2° for semisextiles. The five major aspects get 2° more when the Sun or Moon is involved.
The orb is also a rule that traditions set differently. A tighter orb gives fewer aspects, and a wider one gives more. Hellenistic astrologers also counted aspects by sign: signs a trine apart "see" each other. An applying aspect is still closing on the exact angle and a separating one has passed it.
5 in this chart, tightest first.
Try it on your chart
List your aspects
Your chart draws each aspect as a line and lists it with its orb, so you can see how tight it is.