Buying Adobe Creative Suite 6 (there are more than one version) means buying a perpetual license to a collection of CS6 software (e.g., Master Collection would be all the CS6 apps). You have the software as they are now, but you can use them as long as you like without further payment. When CS7 comes out, you'll have to pay the upgrade price to get the next versions. Or stick with what you have.
If you buy a subscription to the Creative Cloud, you'll pay a subscription fee (either month-to-month, or cheaper for a yearly license) which will get you the latest versions of CS6 Master Collection applications, plus additional applications like Muse, Edge, Lightroom, plus additional services like TypeKit. Also you'll get between-major-release updates to all those applications. If you keep paying for your subscription beyond this year, you'll also be able to get upgrades to CS7 when it comes out at the same subscription price. But if you stop paying for your subscription, you'll lose the right to use it.
What Steve said.
In a nutshell, it's a choice between: perpetual license vs perpetual payment.
Perpetual license = big upfront cost but you can use the software forever. When CS7 is released, you again pay a big upfront cost to upgrade (or stick with CS6). Etc.
NOTE: Commencing with CS6, you can no longer skip versions when upgrading. When CS7 is released, you must upgrade from CS6. If not, you stick with CS6 forever (and that's OK). When CS8 is released, you must purchase it in full (since CS6 will not be eligible to upgrade beyond CS7).
Perpetual payment (Cloud) means always paying the monthly fee for as long as you use the software. There's no extra cost for CS7, CS8 etc. You just keep paying the monthly fee and the upgrades are included in your membership.
Towards comparing the differences and making a decision between the two, you may also find this guide helpful:
"Perpetual license vs. perpetual payment" is a good summary... There are cases where each model serves customers best, but ultimately it depends a good deal on individual factors for which one is the better fit.
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