They provide a mechanism for creating instances with the required state and methods. A Class has constructors which allow you to create objects. For example, a Point Class would be used to create aPoint objects. Scheme handles this rather nicely since a function can return a function, and we are representing classes and objects as functions. All Classes must have constructors in order to have instances of that class.
Classes can also have varying methods in which to manipulate the data.
Destructors are made to clean up dynamically allocated memory. In Scheme you have a nice little garbage collector so you don't have to worry as much about these kinds of problems. Classes can also have their own variables. They are known as Class variables which seems like a fairly good name for them. Instance variables would be the variables that each object has for itself.