Can I use iCloud to backup my whole laptop/Macbook Pro, almost 500GB?

No. Apple's iCloud isn't a Mac OS X backup service.

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1. Apple doesn't provide any software designed for this purpose, and some items, such as an iPhoto library, cannot reliably be stored there.

2. It's to share documents between multiple devices, and for iOS backups.

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Basically, it isn't.

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So it is not possible to upload my HD to iCloud? I don't have an iphoto library, I just have a folder with my pictures.

You are asking about iCloud Drive, right? It looks like a drive in the Finder sidebar, and you can drag and drop documents there, as long as they are smaller than 15GB. Your files will upload to iCloud and be stored there. See this link: iCloud Drive FAQ

But there are two things to consider:

• iCloud is meant for updating the documents across all devices. For each and every document you are storing in iCloud, a shadow copy will download to your Mac as well. If you use the 500 GB for iCloud Drive in iCloud, you will need the same amount of storage locally on your Mac.
• iCloud is meant to store the documents you are working with. If you edit or delete change a document on iCloud Drive, the changes will propagate to all devices and update the documents there. This is not suitable for a backup, because you cannot restore the originals from iCloud. A document you delete on modify on your Mac will be deleted from iCloud or modfied in iCloud as well.

You need a backup of all documents you store in iCloud. As Niel said, iCloud can be used to backup mobile devices, if you do not have a Computer to backup your mobile devices (iCloud: Backup and restore overview), but it does not backup Macs. A backup is a copy of your documents, that you never touch or modify, and keep for an emergency.

jules105 wrote:

I just was thinking of using it as a backup,

It is not a backup system, it is synchronous (bad idea for backup)

Use a real backup system, Time Machine is extremely good and it is already on your Mac

Does this mean that to store 500 GB in iCloud, I would need 1000GB of HD space?

The 500 GB in iCloud would use additional 500GB on your system drive in the Library in your Home folder, yes.