If I turn off Photo stream will I lose photos?
Short answer: No, you won't lose your photos.

Solution: For free and complete syncing, the only way to go is to turn off iCloud Photo Library and My Photo Stream and stick with manual wired or WiFi syncing. It's very important to remember to turn off My Photo Stream as well if you're choosing this option. Otherwise your photos will duplicate.

Details:
Turning off photo stream will not make you lose your photos. It will only disable the Photo Stream function, which is mostly comprised of the following:


It sends optimized (device specific) versions of the photos you take to every Photo Stream enabled device.
It includes the last 1000 photos you've taken, or the last 30 days of photos, which ever is greater.
Data from Photo Stream is a one time push only, meaning it won't push again if you delete a photo from your Photo Stream.
It does not take up iCloud photo space (contrary to most people's opinion). It is a separate and completely free service within the iOS/Mac ecosystem.


When you say iPhone 7, I'm assuming you're talking about iOS 7. In iOS 8.3 and above, iCloud Photo Library (if turned on) replaces the need for Photo Stream, although Photo Stream is still available.

Using iCloud Photo Library is great if you have tons of space in the cloud, since it syncs your albums, edits, share activity... pretty much everything. SERIOUSLY, it's sweet as sugar. BUT, realistically, you will only have that kind of storage space if you pay for it; most likely, you'll run out of your free 5GB allotment since it backs everything up.

If you don't mind paying, it's a great solution. But you want to go completely free, stay away from it like the plague.

Here's a great article that explains iCloud Photo Library in excellent and digestible detail: https://tidbits.com/article/15584