I installed Dropbox on our Macs, Windows and Linux machines. It is useful, but insecure. Do not use Dropbox for private or valuable files.
It gives me a folder tree that
To use it, I open the Dropbox folder on my machine, and copy files into the folder, or from the folder to somewhere else on my machine.
The dropbox software puts an icon in the menu bar so I can find the dropbox folder easily. Inside the dropbox folder, I can set up sub-folders that I share with other people I choose. I create a sub-folder and then right-click on it and choose, which opens a web page that asks for the other users' mail addresses. They get mail with a link: when they click on it, the sub-folder appears in their dropbox on their machine, and its contents are copied to their machine.
My wife and son have set up a shared folder in their dropboxes, so that he can send her large numbers of photos for a website she is creating. He just drags the pictures into a folder on his machine; she gets a notification there are new items, and can move them to her desktop.