Since ancient times, the prayer has been defined as:

* "Pious elevation of the mind to God" (Juan Damasceno),
* "The man speaking with God",

* "Your prayer is a conversation with God. When you read, God is speaking to you; when you pray you are speaking to God" (S. Augustine, In Ps 85 en 7, PL 37, 1086),
* The prayer is an act in which the praying individual recognizes God's  supreme domain over them,

Only beings that were created can pray.  Holy Persons cannot actually pray due to their holy nature.  God's Son prays as He has human nature (cfr. Jn 14,16).  The Holy Spirit "intercedes for us with groans that words cannot express", (Rm 8,26).  This is what urges us to pray.

When the prayer is said silently, we refer to it as mental prayer.

If our mind's elevation to God is also expressed in a sensitive way, we have the vocal prayer.  However, this prayer cannot remain only in words.  There must be an inner devotion, so that an empty prayer comprised of only words is avoided.

Within the vocal prayer, the Church distinguishes the public prayer, which is said in the name of the Church by people who are prescribed consecrated for this work; and the private prayer. 

Both kinds of prayer are very important and necessary:   "The Christian is  called to pray with his brethren, but he must also enter into his chamber to pray to the Father, in secret(Mt 6,6); yet more, according to the teaching of the Apostle, he should pray without ceasing (1 Tes 5, 17)" (See, II Vatican Council, S.C., 12).