Camera Roll are photos you have taken with the camera on the iPhone
. Photo Stream is the name for photos that are synced between all devices using iCloud
If you delete pictures from the camera roll, and haven't synced them to the laptop or Photo Stream, you will lose them forever.
Here is the features page from Apple, all about Photo Stream:http://www.apple.com/icloud/features/photo-stream.html
Here is the FAQ from Apple:http://support.apple.com/kb/HT4486
Both links have very good information on Photo Stream, and what it does.
You would like to save on space, and to keep some photos on your iPhone, but not all of them. Photo Stream is a way of sharing photos between devices, but it's not a way of storing them. As it shows in the features, Photo Stream will store 1,000 photos each for up to 30 days.
Therefore to maintain a full library of all the photos you want to keep, you should store them on your laptop. Photos you take on your iPhone go to your camera roll, and they don't get removed after 30 days, and the only limit is the space on your iPhone. You should transfer those to your computer any time you want to remove some and free up some space.
Photo Stream gives you access to recent photos, but of you want permanent collections available online you can create a shared photo stream. Those are accessible to anyone you choose to allow, not just your devices, and are only removed if you remove them manually.
This probably leaves you with a lot of questions, please let me know how I can help. You can keep some photos on your phone permanently in the Photos App, but you should transfer all from your Camera Roll to your laptop as the main storage for all of your photos.