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If your goal is to transfer all files from local_dir the * wildcard does the trick:

$ scp ~/local_dir/* user@host.com:/var/www/html/target_dir

The -r option means "recursively", so you must write it when you're trying to transfer an entire directory or several directories.

From man scp:

Recursively copy entire directories. Note that scp follows symbolic links encountered in the tree traversal.

So if you have sub-directories inside local_dir, the last example will only transfer files, but if you set the -r option, it will transfer files and directories.