What is filioque?

Ben Myers has a fascinating (and appropriately hard hitting) post, Why I (still) confess the filioque?
Unfortunately, Ben never defined filioque. However, it became clear from the context and the wikipedia entry:

Filioque, Latin for "and (from) the Son", was added in Western Christianity to the Nicene-Constantinopolitan Creed. This insertion emphasizes that Jesus, the Son, is of equal divinity with God, the Father, while the absence of it in Eastern Christianity emphasizes that the Father is the only one cause of the two other persons.

Et in Spiritum Sanctum, Dominum, et vivificantem: qui ex Patre Filioque procedit.
(And in the Holy Spirit, the Lord, and giver of life, who proceeds from the Father and the Son.)