Evangelism is the Christian practice of preaching about Jesus Christ to both Christians and non-Christians. The purpose of most evangelism is to help others receive eternal (forever) salvation to people who have not heard of, or have denied God. Evangelism is done in obedience to the Great Commission, a command from Jesus to his disciples to convert others, as recorded in the New Testament. 

Christians who are superior at evangelism are known as evangelists, whether in their areas or to far away places. Some Christian traditions consider evangelists to be in a leadership position, and they may be found preaching to large meetings.

Notable evangelists 

Charles Finney was an evangelist in the 19th century. He was a leader in the Second Great Awakening, a time when many in America came to the Christian faith. Dwight L. Moody preached in the last half of the 19th century and also started a Christian school, the Moody Bible Institute. Aimee Semple McPherson  preached in the 1920s and 1930s. In the middle years of the 20th century Festo Kivengere, an Anglican Bishop preached in Africa with great results. A prominent evangelist in the 20th century was Billy Graham. His son Franklin Graham followed him, preaching in the 21st century.  Reinhard Bonnke (1940-2019) was an evangelist, largely in Africa. He preached to as many as 100,000  persons at a time. Many missionaries are evangelists, although missionaries also have done other work to promote their faith.

Types of Evangelism

Preaching can be done in churches or in special meetings.  Well known evangelists can draw crowds of many thousands of persons to hear their message.

Literature is used to spread the message.  Small papers with a one or two page message are called "tracts".  Some Christians give these to friends or even strangers. Many books have also been written to spread the Gospel.  Some are overtly Christian and others are less obviously so. C.S. Lewis has written extensively using both genres. The translation, printing and distribution of the Bible can also be a form of evangelism. 

Personal Evangelism refers to people simply talking one on one to others about their faith. 

Radio is widely used to present the Christian message. 

Television has been used to broadcast both preaching and dramatic programs with an Evangelistic message. 

Film Many films have been made with a Christian message.  "Jesus" was a film of the life of Christ based on the Gospel of Luke. It was funded by Bill Bright's Campus Crusade for Christ. It is probably the most watched  and most translated film of all time. 

Christian Web Sites are often Evangelistic. Many Churches have Facebook pages or web sites with a link to something like "How to find God".

Music is used to attract listeners to hear a message.  Often the message of the song itself is a form of Evangelism.  Amazing Grace is a song with a Christian message that has found its way into popular culture. The Salvation Army has long used brass bands playing in public places to attract an audience to hear the gospel message.

Medicine is sometimes used in Evangelism.  Many Hospitals were founded by Christians to serve the physical needs of the public and at the same time showing them Christian love. This often leads to patients asking about the meaning of life and suffering. 

Healing through prayer is sometimes promoted as a way of attracting interest in the Christian message.  It is recorded in the Bible that Jesus often did this.  Modern "faith healers" have been criticized for making false claims about persons being healed. But the practice is still common in Pentecostal circles. Catholic and mainline Protestant churches sometimes have healing services also. 

Social programs are used by many churches to help persons in need of food, clothing and short term housing.  Some churches do this without any attempt to "preach" to those receiving the benefits.  Others, such as some "Rescue Missions" may require attendance at preaching meetings to receive the help.  

Education has historically been a tool for Evangelism.  Many famous schools were founded to help spread the Christian message, even though many of them became secular over time. 

Although most Christian denominations perform some sort of evangelism, Evangelical and Pentecostal denominations are very focused on this issue.

References 

Christian theology
Evangelicalism