Pointers of non-existent classes in C ++, how does it work?

Sometimes I run into situations like this:

#ifndef CRIARVENDA_H
#define CRIARVENDA_H

#include <QDialog>
#include "cliente.h"

namespace Ui {
class CriarVenda;
}

class CriarVenda : public QDialog
{
    Q_OBJECT

public:
    explicit CriarVenda(QWidget *parent = 0);
    ~CriarVenda();

private:
    Ui::CriarVenda *ui;
    Cliente *cliente;
};

#endif // CRIARVENDA_H

in this case

namespace Ui {
class CriarVenda;
}

It is created a kind of a prototype of a class that will still be created which does not make much sense.

In this case it still does not make sense yet, where it creates a "Prototype class"

#ifndef CPU_HPP
#define CPU_HPP

#include <array>
#include <cstdint>
#include <functional>

class GameBoy;

Create a pointer to it

GameBoy* gameboy;

    public:
        CPU( GameBoy& core );

#include "GameBoy.hpp"
#include "CPU.hpp"
#include "RAM.hpp"

CPU::CPU( GameBoy& core ) :
    gameboy( &core ) {
    initOpcodeArray();
}

std::uint8_t CPU::readMem( std::uint16_t address ) {
    gameboy->ram.readMem( address );
}

void CPU::writeMem( std::uint16_t address, std::uint8_t value ) {
    gameboy->ram.writeMem( address, value );
}

Then call up the members of the class.

What is the name of this? I'm looking at some C ++ books I have and I do not see anything like that, and what's the use?