Muggles' Guide to Harry Potter/Magic/Floo Powder

Overview
Floo powder is a greyish or silvery powder which allows access to the Floo Network transportation system.

Extended Description
The Floo Network is a magical linking of most fireplaces in the Wizarding world; it is used as a means of rapid transportation from one Wizarding building to another. Floo Powder provides entry to that network; place a pinch of Floo powder in the fire, and it becomes green and harmless. You may then enter the fire, and speak your destination, and you will be carried to the fireplace you have named. It is a very dizzying ride; you spin very rapidly while in transit, and you would be well-advised to keep your elbows tucked in.

It is also possible to place only a body part, such as a head, in the fire, whereupon that part will be the only thing that is carried to your named destination. Placing your head in someone else's fireplace is one way to communicate over long distances, but as Harry finds out, it can be rather disconcerting to have your head spun around rapidly while your body stands still. Colloquially, this is sometimes referred to as a Fire-call.

Apparently in certain circumstances, Floo powder or something similar to it can be used to summon someone. Professor Snape uses this in Harry Potter and the Prisoner of Azkaban to summon Professor Lupin to his office.

Analysis
Most commonly, after you toss a pinch of the powder in the fire, you step into the fire and pronounce the name of the place you want to go to (e.g. "Diagon Alley"). If you only want your head transported you sit in front of the fire and only stick your head in. Unclear pronunciation has its consequences; you might end up somewhere other than your intended destination.

However, if you only want to call someone, it seems that it is unnecessary to say the name of that person's actual residency, the name of that person will do. It is unclear whether you need to know their current whereabouts, though.

Questions

 * 1) When someone is talking to another person with Floo powder, their head is in that other person's fireplace. If someone was in the same room as you, talking to someone else with Floo powder, what would you see?