Name Magic
Names have significant mystic importance in folklore, and this is reflected in Ars Magica. There are several applications of this concept.
At the most basic level, a person's nickname or birthname can be used by a magus to increase penetration through sympathetic magic (see ArM5 p. 84). Since this requires an arcane connection, it is not trivial to apply and confers only a small benefit.
Names of beings of Might are more significant. The full name of an airy spirit forms an arcane connection of indefinite duration to it (TMRE p. 29, RoPM p. 110), which allows the magus to summon it as well as a +5 penetration multiplier. More generally, any "intelligent entity with a personality and a Might score" (which, arguably, includes just about any being of Might that will be of significane in the saga) has a True Name (RoPM p. 110), which confers the same benefit but is harder to learn (requiring experience points in the relevant (Realm) Lore). This includes angels and demons, as well as many Magical spirits (whether Faeries have a True Name remains unclear).
Finally, Hermetic Synthemata are even more occult names. Any being with Might has synthemata, and they can be learned by theurgists (TMRE p. 86) as MuVi spells. Synthemata essentially assure Penetration, but are hard to learn and may be somewhat redundant considering the efficacy of a True Name.
In addition, some magi can invoke Names of Power to bolster their magic. While these names call upon powerful daimons and occult entities, they have hold no power over them.