Quote

"I know not with what words I can, or ought, to speak of these pure spirits, for they have neither hands nor feet, neither image, nor form, nor substance; neither can we understand the nature of their being; so how can we speak of them?" - Johannes Tauler, Sermon XXIV.

Teaching

According to Father Jean-Michel Garrigues, O.P.:

Angels are intangible creatures, purely spiritual. They are not located in a given place, unlike corporal creatures. They are present where their assignment as messengers of God sends them. 

Each angel is unique. He contains within himself all the richness of his specific nature, unlike men who all belong to the same species: the human species. 

Good angels are "in Heaven," which means that they are blessed by the constant presence of God. God grants them superior knowledge which allows them to understand intuitively, immediately. However, they aren't aware in advance of God's plans, which are progressively revealed to them (cf. Ephesians 3,10 and 1 Peter 1, 12).

Prayer

Ye angels, so holy and so pure, spirits truly blessed, who stand before your Lord, you who are those brilliant stars which shine with so much luster in the empyreal Heaven, pour into my soul, I beseech you, your blessed influences, preserve my faith in its purity, my hope in its firmness, my virtue in its integrity, and make me advance ever in the love of God and my neighbor, that after this life I may merit with you to contemplate the sovereign beauty of the Heavenly Father. Amen. 

In Devotion to the Nine Choirs of Holy Angels, translated by E.H Thompson, 1869.