What is the difference between except and besides? When do we use each of them?
They have totally different meanings, in fact opposite in some sense. Except serves the purpose of exclusion while besides serves that of inclusion.
Suppose John is in a class of 30 students. If the other 29 students are going to the zoo but John is not, we say
The whole class is going, except John.
Now suppose Mary from another class is also going to the zoo. Then we say
Besides the class, Mary is also going.
Sometimes they have the exact same meaning.
See this other post: What is the difference between "apart", "except", "other than" and "besides"?