My understanding (erroneous as I may be told this is) is that iCloud is like a virtual external drive to some extent. Maybe that is my first mistake. But before everyone starts saying to look at other posts, please know I believe I have read all the posts about photo stream, camera roll, and the link that was posted (in one of these answers on this general topic) about iCloud backups, as well as those Q&A's related to freeing up space. I am still confused. 

I am reaching storage capacity on my iPhone. I have a lot of photos on my phone because they've always migrated over from phone to phone every year or so. However, I need to free up some space and would like to delete photos from my iPhone (the device I took them on), but am still not sure if I do this, if it will remain on iCloud.

I have an iPad and a Macbook too, and they (the photos and docs) are on there, but with photos I just don't ever want to lose, I want to be sure that if I start deleting off my phone, and my computer gets stolen or dies and my iPad is broken or whatever, that I can still always access all of my photos by just logging in to iCloud from any device.

I (supposedly) have iCloud backing up all of these things. However, If I log into iCloud.com, I do not see my documents and photos. Shouldn't I conceivably be able to log in to iCloud.com from any device (Apple or not - say my work computer) and access my saved documents and photos?

Because I cannot go to iCloud.com (not settings/iCloud - bit iCloud.com) and see these things, I am hesitant to delete photos from my phone, depending on the idea that they are stored in iCloud. The same is true, to a different extent with my documents.

I have purchased extra storage on iCloud w/the idea that these things will always be there. What am I missing.

Also, aside from apps, photos and documents - are there some less obvious things I can delete that will free up space that would not be evident to the less than professionally savvy user?