Rewinding the clock a few years back, I didn't use Dropbox all that often - even the 'cloud', I never used often. I relied mainly on my Google account, and my QNAP NAS. When I started here at TweakTown, I needed something where I could access all of my work, on any machine, on any operating system, anywhere.
I jumped into Dropbox when I purchased by Galaxy S III, as Samsung were handing out 50GB of free Dropbox storage - so I thought I'd use it. From there, I've uploaded tens of gigabytes of data to the cloud, and have it synced across multiple machines, operating systems, and even on my NAS.
Your first step, of course, is to ensure you have a Dropbox account - if not, you can sign up here.
I'm going to presume you're using Windows here, so once you're signed into your Dropbox and have some files synced, you'll have a 'Dropbox' folder in your Users directory. This folder, is the folder that everything is stored in, and you'll access this constantly.
The best way to ensure your data is on multiple machines, is to make sure you have Selective Sync enabled. Selective Sync will allow you to sync just the folders you want and need, and not absolutely everything.
Thanks to Dropbox working across virtually all platforms, it means you can have a synced folder between your Mac, and Windows. This comes into play when I'm travelling, as I can move with my MacBook Air, with the ability to edit anything and everything in my Dropbox folder. Once I get home, it's all synced on my PCs.
In order to copy the folder out of your PC and onto something like a NAS or external HDD, is to navigate to the Users folder, copy the Dropbox folder, and paste it onto your chosen backup.
I have this setup so that I have a "work" folder, a "personal" folder, and so on. My work folder contains all of the reviews I've written, so that whatever machine I'm on, I can be productive. I can quickly pull up my Word document, type up a bit into a review, save it - and it syncs to Dropbox.
Alternatively, I can open up an image, resize them, edit them, and again - sync them. Once this is synced, every machine I turn on will quickly download the new, or edited files, and I have a constant back up of everything I've done.
I find this is the best way, as it means I have a 100% backup online, another 100% backup on my main Intel NUC machine, on my Apple MacBook Air, and on my QNAP NAS. Every month or so, I backup to my NAS just in case - it's not needed, but the more backups, the more secure, right?