They define objects. Give me a more specific question and I might give you a more specific answer.
Classes divide your work in small chunks. For example you want to write a management for a university system, so in university there are teachers, students, staff.So you make a Teacher class that accept teacher data and another class should be student class that accept student data and so on...... :)
Consider classes as representations of "things", such as animals, boats, people, etc. Each class will consist of member variables that determine the uniqueness of that object, such as for people that may be name, address, age, etc. as well as methods to set/get the values of those variables such as setName/getName, setAge/getAge, etc. This allows us to view and manipulate our digital world in a rational manner.