There are several ideas of what an evangelist is, but looking at Scripture will help clarify any misunderstanding.
First, simply observing the meaning of the word provides a definition of what an evangelist is, and that's a herald of good tidings (in biblical context, this good tidings is the Gospel). So by definition, an evangelist is simply a Gospel preacher.
Next, looking into the mention of the gift of the evangelist outlines the scope of his ministry. Ephesians 4:11-16 reads:
First, simply observing the meaning of the word provides a definition of what an evangelist is, and that's a herald of good tidings (in biblical context, this good tidings is the Gospel). So by definition, an evangelist is simply a Gospel preacher.
Next, looking into the mention of the gift of the evangelist outlines the scope of his ministry. Ephesians 4:11-16 reads:
Ephesians 4:11-16
11 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
12 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
13 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
14 That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive;
15 But speaking the truth in love, may grow up into him in all things, which is the head, even Christ:
16 From whom the whole body fitly joined together and compacted by that which every joint supplieth, according to the effectual working in the measure of every part, maketh increase of the body unto the edifying of itself in love.
12 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
13 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
14 That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive;
15 But speaking the truth in love, may grow up into him in all things, which is the head, even Christ:
16 From whom the whole body fitly joined together and compacted by that which every joint supplieth, according to the effectual working in the measure of every part, maketh increase of the body unto the edifying of itself in love.
The gifts to the church include two which were transitional and were used in helping the church become established while the Bible was not yet completed. Now that we have the completed Word of God, prophecy is no longer necessary, nor are apostles. However, the preaching of the Gospel is something that is to be done until the end -so the gift of the evangelist is still very valid. Each of these gifts have different roles, but all of these accomplish the same task. They are given to help instruct, equip, and edify believers in order to unify and protect the church from false teachers and their doctrine. This means the evangelist has a ministry to believers as well. This is because the Gospel not only means salvation from sin's penalty for unbelievers, but it also provides access to victory over sin's power after salvation to believers. In fact, Romans 6:14 tells us "For sin shall not have dominion over you: for ye are not under the law, but under grace." Upon receiving Christ as Savior, believers inherit a new Father, a new life, a new future, and are no longer subservient to their old master of sin. We're told, "Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new" (II Corinthians 5:17). The Gospel message goes beyond just inheriting this new life, but living in it. Just as Paul powerfully states in Galatians 2:20b, "and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me." This is how the Gospel message continues its impact to believers.
Finally, studying the examples in Scripture provides insight for the method of evangelism. There are ninety examples of the work of the evangelist being done. If reading in the Greek, it would appear as the verb form of εὐαγγελιστής (evangelist) which is εὐαγγελίζω (to herald good tidings or to evangelize). This is translated into our Bibles a variety of ways, typically using a form of the word preach. In these examples, some are itinerant as they share the Gospel, some are sharing the Gospel locally, some are preaching to large audiences, some one on one, and some were planting churches. While opinions as to who some of these people were may vary, the fact that they were preaching the Gospel (doing the work of an evangelist) can be agreed upon. With the absence of a directed method, and the presence of such a variety of examples, it can be concluded that there is no limit as to how the Gospel is dispersed. The evangelist is then responsible for preaching the Gospel through whatever method God has placed on his heart (keeping in mind that the Bible is observed where it does speak clearly on subjects like Biblical authority, morality, etc.). In fulfilling this work, the provision for the evangelist as being a Gospel preacher is clarified in I Corinthians 9:14, "Even so hath the Lord ordained that they which preach the gospel should live of the gospel."
The evangelist then is a preacher who declares the impact of the Gospel to both sinners and saints through the venue God has made available to Him. We seek to engage in whatever opportunities the Lord provides in order to spread the Good News.