New Mac User

iPhoto copies every files from the SD card/External Hard Drive/Other storage media (Internal/External/Local/Server/Web) to a library. It's on your HDD/SSD.

Photostream copies every picture on your iPhoto library, pastes them into the cloud, then pushes them to your other devices.

There's a delete button in Mail.

 

Yes, delete the original photos, they remain in iPhoto

No, it's not just about downloading. It's about mirroring. Make a change here, it goes there. Make a change there, it goes here. It's about your work going with you and not having to manually sync them together.

In this case, when you added photos in the Photostream, it also added photos in to your other devices. When you deleted photos there, it deletes everywhere else. When you take a picture here, it appears everywhere else.

 

If you delete photos from the HD that you put in iPhoto, they will stay in iPhoto. But if you delete photos in iPhoto, they will be wiped from your HD too (assuming they were only imported to iPhoto originally)

I just put all my photos into iPhoto, not the HD, because like I just said, when you put them in iPhoto, they will go on the hard drive (albeit the iPhoto folder in the HD). Also, say if you import 2 GB of photos into iPhoto, maybe ~2.5GB will be stored on your HD, because 0.5GB is used to make thumbnails and faces previews. If you try to delete the thumbnails, they'll just come back when you open iPhoto because iPhoto will index them again.

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You need to set GMail to enable POP (or POP3, I forget which it is). Then set up your Mail account as POP. Basically, IMAP copies what's on your server and vice versa, which is convenient for most people, but also takes up a lot of space on the server or your mac because it's an all-or-nothing approach; The mail is either everywhere or nowhere. But POP will give you independent control over your mail on the server and your Mac. With POP you could receive an email on your Mac, but delete it, and it'll still be on the server.