A focus is an object used to aim and focus magic. Unicorns have horns which are a natural focus.
Objects enchanted to be a focus rarely have other enchantments on them, as stacking enchantments doesn't often work out well. Pointed objects like wands or daggers make for a good focus, as they make it easy to aim. Spellbooks too make for handy focuses, as you'll always have a list of spells and the thing needed to use them in once place. Emerald's Spellbook is an example of this.

Historically goats made focuses out of deer horns. They did not realise they could use other things, and have caused some lingering racial tensions because of this.

Emerald Jewel also has another focus, one with an unexpected design.