Shell redirects

When you invoke curl from a shell or some other command-line prompt system, that environment generally provides you with a set of output redirection abilities. In most Linux and Unix shells and with Windows' command prompts, you direct stdout to a file with > filename. Using this, of course, makes the use of -o or -O superfluous.
curl > example.html
Redirecting output to a file redirects all output from curl to that file, so even if you ask to transfer more than one URL to stdout, redirecting the output will get all the URLs' output stored in that single file.
curl > files
Unix shells usually allow you to redirect the stderr stream separately. The stderr stream is usually a stream that also gets shown in the terminal, but you can redirect it separately from the stdout stream. The stdout stream is for the data while stderr is metadata and errors, etc., that are not data. You can redirect stderr with 2>file like this:
curl > files.html 2>errors