How do I find a file on http://url_path/filename

[HP Wei]

I am a bit lost in the huge amount of docs.
(It will get better when I am exposed to it for a while.)
For now,
Please tell me where I could find the module for doing:

(1) check if a file exists on http://url_path/filename?
(2) retrieve that file to a local disk.

thanks,
HP