correspondence
Correspondence means that the school sends you study materials by mail. Usually, you study at home by yourself. Before the end of the term, you will be taught several times in person, and then you will have a unified exam. It is suitable for office workers and candidates with less spare time.
layman
Generally, students are enrolled in colleges, and classes are arranged at night or on weekends. Therefore, candidates from different places or on-the-job people are more suitable.
Free from production or daily work and undertake other tasks.
Full-time study is full-time study at school. This way of learning is more suitable for students, because it can't make working people take care of both work and study.