The zodiac is a band of sky divided into twelve equal sections of 30 degrees. Each sign is named after a constellation the Sun appears to pass through during the year, and each one has its own element, modality and ruling planet.
Because the signs follow the seasons, they also follow a story. Aries begins the cycle with fresh energy at the spring equinox, while Pisces closes it with reflection and compassion. Reading the signs in order shows how each one builds on the last.
Your Sun sign is only the start. Most people have planets spread across several signs, which is why two people born a week apart can feel very different.
Further reading: the zodiac.