Photo Stream only saves either your last 30 days' worth of photos or your last 1000 photos, whichever you hit first. However, in iPhoto on the Mac, the "automatic import" feature (which I believe is enabled by default) will automatically copy any photos in your photo stream to your Mac's local hard drive.
So I think selecting "Photo Stream" in iPhoto shows the temp store, but they are also saved into a Event where they will remain even after being removed from Photo Stream.
Correct (again, as long as you have automatic import enabled. You can double-check the setting in iPhoto preferences).
Photostream will store the previous 30 days worth of photos automatically. That means, every photo you take on your iOS devices will be sent to Photostream, but only the last 30 days worth of photos will be available there.
If you enable 'Automatic Import' of Photostream in iPhoto (Preferences > Photostream > Automatic Import) then when you launch iPhoto, it will automatically download a copy of any photos that are in your Photostream, and automatically create an Event titled: {MMM YYYY} Photo Stream (eg. "Jul 2012 Photo Stream")
This Event will be automatically kept up to date, as long as iPhoto is open long enough to sync everything required (depends on your connection).
In effect, as long as you open iPhoto once every 30 days and leave it open long enough for your Photostream to be imported, you should never have to manually import a photo from your iOS device at all.
As for removing items from your Camera Roll on your iOS device, as long as you see that they are present in Photostream (or in iPhoto), then it is safe to delete them from your device. (There is no way around this but to do it manually as far as I know.)
Currently, Photostream does not support videos. To import videos, connect your iOS device to iPhoto and import these manually.
As a side note, in order to keep some photos on the device, you can create a smart album in iPhoto along the lines of "Photos taken within the last 30 days" or "Photos rated 4 stars or more", and configure iTunes to sync these to your device. That way, you can be sure that no matter if you clear your Camera Roll on your device, or delete certain photos, you maintain some convenience.
Best Answer
If you have Photostream turned on and set it to download Photostream photos, then iPhoto will put them in an Event by month. It will look similar to other photo Events that you've created. iCloud Photo Library sees these as just normal photos and will upload them. Everything in the System Library (which means the default Photos library) will get uploaded, so the only way to stop it is to turn off iCloud Photo Library or put those photos into a separate Photos library.
If you have iCloud Photo Library turned on in your Mac and iPhone, then they will eventually sync up.
All pictures in your iPhone Photos app will get uploaded into iCloud Photo Library (including screenshots and saved photos from other apps/internet).