Basically Adobe is shifting away from selling boxed products, instead selling applications by subscription. Once you subscribe, for a monthly fee, you can then download the latest versions of the applications you are eligible for under the subscription plan you chose. As long as you keep paying the fee, you can carry on using the software. Once you cease your subscription, then at some point Adobe will block your use of that software, until or unless you re-subscribe. That 'at some point' is currently 90 days but this is due to increase to 180 in the near future.
Adobe use of the phrase 'Creative Cloud' (CC) has caused a bit of confusion. To be clear, the applications bought through CC are downloaded and installed on your computer, and run on it just as if you had installed from DVD/CD. The application then periodically checks with Adobe that your subscription is still valid. To get around the issue of users being away from an Internet connection, there's effectively a 90/180 day grace period. So, plenty of time to go on an 3 month assignment then (are you listening National Geographic?).
I use two Adobe Products, Lightroom and Photoshop. Lightroom will continue to be sold as a boxed product according to Adobe, but Photoshop is moving to CC. I thought I had foolishly missed the opportunity to upgrade from CS4 to CS6, but in hindsight this may have been a wise move, because for less than £9 a month, I have now not only been able to upgrade to CS6, but will also be able to move to the next version, Photoshop CC, when it becomes available later this year, for no additional cost.
Given that previous upgrade costs have been around £180, short term I'm quite happy with this. It remains to be seen what the cost will be in year two, as Adobe is currently giving introductory discounts to encourage users to subscribe to CC. For now though, I'm happy to be running the latest version for the next 12 to 18 months.