An orb of shielding is a polished, spherical chunk of crystal or stone aligned to one of the planes of existence. If you're a spellcaster, you can use this orb as a spellcasting focus.
If you're holding the orb when you take psychic damage, you can use your reaction to reduce the damage by 1d4 (to a minimum of 0).
An Arcane Focus takes a specific form and is bejeweled or carved to channel arcane magic. A Sorcerer, Warlock, or Wizard can use such an item as a Spellcasting Focus.