Don't use iPhoto. It's not for everybody and, given your preferred usage, it's not for you.
Quick definitions:
A Managed Library, is the default setting, and iPhoto copies files into the iPhoto Library when Importing. The files are then stored in the Library package
A Referenced Library is when iPhoto is NOT copying the files into the iPhoto Library when importing because you made a change at iPhoto -> Preferences -> Advanced. (You unchecked the option to copy files into the Library on import) The files are then stored where ever you put them and not in the Library package. In this scenario you are responsible for the File Management.
Yes you can set iPhoto to reference the files in their current location. But iPhoto is not good with referenced libraries - they mean more work for you for exactly zero benefit - and if there is an issue, especially when the Library is on one volume and the photos stored on another, you could find yourself reattaching the individual files in the Library to your actual photos, one by one, for the entire collection.
See this for more: https://discussions.apple.com/thread/3062728?tstart=0
It'll tell you how to do it and why you shouldn't. Some of the material is out of date now, but the argument is the same, essentially.
There are no such reservations about Aperture, iPhoto's bigger brother, which has the tools to comfortably manage referenced Libraries.
Yes, Windows Explorer is the the Finder on OS X.
You can import from a camera to a preferred location using the Finder or Image Capture (in the Applications Folder)
You can run a managed Library (the default setting) from an external disk very easily, but the disk needs to be formatted Mac OS Extended (Journaled).
As an FYI:
Is there a way to simply have iPhoto pointed/looking at my external harddrive without having duplicates, or loosing my current file structure?
The last element "or loosing my current file structure" pretty much indicates the conceptual difference between iPhoto and what you're used to. With iPhoto your current file structure is of little or no relevance. Why? Because you never access the photos that way. The point of iPhoto is that it replaces the Finder (Windows Explorer) for anything to do with your photos, It's the go-to app for everything.
Best analogy I can come up with: The Contacts app for your addresses and phone numbers. Here's an app that manages your contacts in one single location and makes them available to all the other apps on your computer for whatever reason you might need them.
Well iPhoto is an app that manages your photos in one single location and makes them available to all the other apps on your computer for whatever reason you might need them.
So, want to edit the photo, start with iPhoto. Edit it with iPhoto, or any editor you prefer:
You can set Photoshop (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in Photoshop or your Image Editor, and when you save it it's sent back to iPhoto automatically. This is the only way that edits made in another application will be displayed in iPhoto.
Want to email it? Use the Media Browser to get it into any email app or service you prefer... similarly to upload it to a website, to use it in a word processing document and so on. Any thing you need to do with your photos is done either with or via iPhoto.
So, if you're going to use iPhoto (or, indeed Aperture) you need to break a connection in your head. That Jpeg is not your photo. That Jpeg is a box, and inside the box is your photo. Your current file structure is an excellent one for boxes, for photos, not so much. It's inflexible and requires you to remember when you took the photo. A photo manager leverages the extra possibilities of the media - metadata, the fact that the data is visual etc - to produce a filing system that's infintely more flexible than date storage.
But if that's not what you want to do, then don't use iPhoto.
By all means post back if you have more queries.