Answer: If you upload your files with the standard upload method, everyone that knows the download URL can also access the file.
Since the URL is a randomly generated number, "guessing" the correct download URL is rather difficult/impossible. Therefore if you give the download URL for a file to selected persons only, it is very likely that only those persons download the file.
In addition, you can optionally specify a password to protect an uploaded file from unauthorized access. Before downloading such a protected file, the password must be entered correctly to actually download.