Speak, Whisper, Shout, Tell

What's the difference between /whisper and using the tell channel?

Is there a way for only speaking without all peoples in the area can hear? I mean... Speaking to everyone in my "perception radius" as shown on the map?

That's fairly hard to do good roleplaying without this...

!ALL - global chat, everyone in the district hears you.
@Guild - Everyone in your guild hears you no matter where they are.
#Team - Everyone in your party hears you.
$Trade - Global like ALL but separate channel.
"Whisper - Only the person you whisper to can hear you.

Nope, there is no way to only speak to people in your map area. It's always zone wide / group / or guild.

Doesn't that bother other peoples in the same zone when other peoples roleplay? Doesn't that get confusing sometimes?

The limited amount of RP I see in this game is usally in group chat or whispers.