obscure c++ operator

Question

Possible Duplicate:
What is this weird colon-member syntax in the constructor?

Can someone tell me what the last line of this code block does? I have looked up the : operator but only found it in a context for variables, not with a function.

class Vec3
{
public:
    float x, y, z;

    Vec3() {}
    Vec3(float _x, float _y, float _z) : x(_x), y(_y), z(_z) {}