Relics are a feature added in the Nook And Cranny Update (0.5.3).
They come in two forms: The conventional forms of relics (Fossils, Statues...) or Messages inside bottles. In each biome, 3 to 5 relics are available. The Messages inside bottles are globals and can be found in every biome
The Relic's primary purpose is to be something for the player to collect, and their descriptions are usually very entertaining. However, there might be other uses for certain relics.
They are collectables and can be viewed in the "Relics" category.
Please beware for spoilers and add any if missing.