The Shadow Orb (Spell) is a summon-type spell, obtained from breaking Shadow Orbs. When summoned, the Orb will follow the player around and project light in a manner similar to the Fairy Bell or Wisp in a Bottle.
However, the summoned orb has low acceleration and a slow top speed, so it often falls behind, especially when the player falls a long distance or otherwise travels at a fast speed (such as when using the Hermes/Spectre Boots). If it falls too far behind, it will warp to the player's location.
Pressing B, the buff hotkey by default, will also activate the Shadow Orb. Be warned that pressing B will also use any buff potions that you happen to be carrying.
The Orb's movement is not hindered by blocks.
Pressing "W" or "S" will make the orb move upwards/downwards.
The only ways to dispel the Orb are either right-clicking the buff icon below your hotkeys, or activating the Fairy Bell (which will replace the Orb).
Sometimes on multiplayer the players will see in your Shadow Orb that the light on it is white and not violet.
The Shadow Orb gives a lot less light when it floats in a wall.
In the Mobile Version, the Shadow Orb is still called the Orb of Light, which doesn't give much more light than the player.