What does a colon following a C++ constructor name do?

What does the colon operator (":") do in this constructor? Is it equivalent to MyClass(m_classID = -1, m_userdata = 0);?

class MyClass {
public:

    MyClass() : m_classID(-1), m_userdata(0) { 
    }

    int m_classID;
    void *m_userdata;
};