how to measure online traffice?

When reading "Web numbers: what's real?", what comes into my mind is how do those companies actually measure traffic? Do they purely use certain advanced technology to count the number of visiting? Is there any methods or measurement design that we learn in the method class used in this process? I think it must have some measurement design or method fixed in the technology, but have no idea after reading it. If we go back to the definition of what traffic is and its complicated relationship with advertising, and then how to measure the meaningful traffic, maybe it'll make more senses to me, or let the academic folks in communication area know how to take advantage of this online method research area.