What makes this book unique is its brevity as well as its gravity. So clear is the purpose by which Jude wrote that he states his purpose at the outset. He shifts his original focus from the “common salvation” to the condemnation of false teachers (vv. 8-10, 16). He, like Peter, pronounces the divine judgment on those who pervert and distort the truth of the Gospel; it is his God given duty to expose and dispose of these men.
I'm going to start off by showing the classic features of Jude as a NT epistle. Then in part 2 I'm going to offer several key factors involved in properly interpreting a letter of this nature; namely, the literary genres.
Let me begin by listing the five points that you can find in Ryken’s book. Then I will demonstrate how each one is present in the book of Jude.
Ryken observes an epistle in the following format:
1. Opening or salutation (sender, addressee, greeting)
2. Thanksgiving (including such features as prayer for spiritual welfare, remembrance of the recipients[s], and eschatological climax).
3. Body of letter (beginning with introductory formulae and concluding with eschatological and travel material).
4. Paraenesis (moral exhortations).
5. Closing (final greetings and benediction) 
Opening or Salutation. Like the epistles of Paul, though not necessarily present in the Johannine epistles, Jude forthrightly introduces himself to his readers. This structure is actually different than what we are used to in contemporary times. Modern letters (or epistles) would tend to begin with addressing the recipient thereof, and then at the conclusion the writer would identify themselves.
Another common point in the opening verses has to do with what I call extended identification. This we see Jude doing when he notes that he is a bond-servant of Christ as well as the brother of James. Readers or hearers of this epistle would have been quite familiar with James; as he was the brother of Jesus. Robert Gundry notes, Thus Jude too is a half brother of Jesus but modestly describes himself as “a servant of Jesus Christ.  Therefore the additional information properly defines Jude’s status and authority.
Thanksgiving. This element seems to be absent from Jude because he delves right into his argumentation. However, he does offer a reference to thanksgiving when he refers to his readers as those who are called; which refer to those privileged to be chosen by God. Notwithstanding, his immediate pronouncement of a benediction to the readers plainly notes his custody of thanksgiving for them.
Body of the letter. What is interesting about the beginning of the body of the letter is the nature of Jude’s intention. He mentions that he wanted to write the recipients about the common salvation. Commentators generally agree that Jude would have meant to write either a doctrinal treaties or a letter on salvation as the common blessing enjoyed by all believers.  However Jude felt it necessary to change his intention of writing in a general benedictory fashion to writing in contention of the faith. 
Closing (benediction). Verse 24 is a perfect example of a NT epistle sharing in common with the others. It’s as if Jude ends the letter as a prayer might end by pointed the entire writing to glory and honor of God. A doxology was a common way in which a writer might blend a benediction with reverence to God; horizontally as well as vertically. These closing words could very well have been fragments of an early hymnal.
 A similar structure is also founding Fee and Stuarts book How to read the Bible for all its worth, 3rd ed. p. 56-57. Though they separate opening and greeting and give 6 total points.
 Gundry, Robert, H. A Survey of the New Testament. Zondervan: Grand Rapids, MI, 4th Ed. p. 491.
 Reformation Study Bible, Ligonier Ministries. editor, R.C. Sproul
 MacArthur Study Bible, Nelsons Bible, editor, John MacArthur.
 The Greek word, evpagwni,zomai [egonizomai] actually means to “struggle in behalf of” and is only found in the book of Jude. It shows Jude having to deal with the issues as Paul did in his letter to the Corinthians.