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.
iPhoto is a digital photograph manipulation software, part of the iLife suite.
Photo Stream is functionality by which when you take a photo on one device, it will automatically appear on all your other devices, without syncing nor sending.
Image Capture is the Application developed by Apple which enables users to upload pictures from digital cameras, scanners, or iDevices.
Best Answer
First, make sure what you're seeing is actually Camera Roll and not Photo stream.
This will display the usage for each section. If it's Photo Stream then you have your answer. If it's actually the Camera Roll re-populating then you should check iTunes and make sure that "Sync Photos from" is unchecked.
If it was checked, uncheck it and do a sync again. That should clear it out.