When I've completed an update to a website on my local server and then want to update this new info to an already existing website, do I just re-upload the sql file, photos, etc or must I re-upload the whole website?
If you update the sql DB, only the contents of the website will be updated (news, forum posts, page.php?# etc...) - that kind of stuff. But if you made changes to your theme (modify layout, design, add new pics etc ...) you need to upload that folder too.