Historical Setting
Thessalonica was a large and important port city. In Acts 16 Paul gets thrown out of Philippi for preaching and from there he goes to Thessalonica. In Acts 17 Paul preaches at the synagogue there. Some Jews responded; other Jews rejected Paul. There was an uproar and the Christians in the city had to spirit Paul and Silas away in the dark of night. Paul continued the missionary journey and ended up in Corinth where he spent almost 2 years.
Eventually, Timothy joined him in Corinth and brought news of Thessalonica, particularly about arguments over what happened to believers when they died.
Author
Most scholars believe this letter was later than Paul, a letter written by an anonymous disciple in the spirit of Paul.