A Focus is an item used to tap into the magic fields when spellcasting. Most people require a Focus to use any kind of magic, although those who are Naturally Attuned only require them for high level spells.
An item must have a strong sentimental value to the caster to be used as a Focus, and does not require any other properties, magical or otherwise. As a result a caster's Focus is generally unique to them unless the item represents a shared experience. A living creatures cannot serve as a Focus, only an inanimate object. Typically it is something easy to carry or to wear as an amulet, although this is primarily for matters of convenience rather than requirement.