A forum is equivalent to a folder which contains a collection of one or more discussion topics. You post to discussion topics and each post within the topic is called a thread.
For example:
Forum: Classroom Introductions
Topic 1: Where were you born?
Thread 1: Alice: I was born in Germany
Reply to Thread:
Bob: I was born in Germany too! Where in Germany were you born, Alice?
Thread 2: Chuck: I am from California, USA.
Topic 2: Why are you interested in this course?
Thread 1: Dan: I love learning about science!