Doctrines, principles, applications
"As teachers and leaders, it is vital that we nourish those we teach and lead by focusing on the fundamental doctrines, principles, and applications emphasized in the scriptures and the words of our latter-day prophets" (Elder Daniel K. Judd, "Nourished by the Good Word of God," October 2007 General Conference).
Doctrines
Doctrines are statements/sentences.
For example, "We are all children of God."
Principles
Principles are not statements/sentences. (They seem to generally be nouns.)
For example, the first 2 principles of the gospel are faith in the Lord Jesus Christ and repentance.
Every principle is a "key word," because pondering a key word "unlocks" one's personal spiritual treasury and suggests to one's mind the associated acquired doctrines and experiences.
Applications
Applications are ways to implement doctrines and principles.
For example, Family Home Evening is one current way to implement the doctrine that parents should teach their children.
Credit to Elijan for this
