Is Photostream a back-up?

So my ipad mini was restore last week via ITunes because my password isn't working.

I'm so frustrated and I swear I remember my pass code because My screen is lock every minute.

The next day, I enter my pass code and it isn't working anymore and there is no way my little brother change it because its locked so my ipad was disabled after a couple of attempts.

Then, I can't backup my files anymore so I decided to restore it since I don't have much important things except for the photos.

When I thought everything was deleted, I check my camera roll and its empty but my photostream still have some of my photos.

As I estimated at least 60% of my photos are still stored , but I didn't copy anything before.

So is photostream a backup? If yes why does it only save random picture? I mean not all my photos are there. If I have 300 , only 100 was there.

And lastly, idk about the connection of photo stream and iCloud?

Help me pls.... Thank you.