In some respects they are the same thing. In others they are different.
Think of the Revenue Cycle as being a part of core Marketo infrastructure. It is the way that Revenue Cycle Analytics/Explorer works. You set up a model, which you can see visually, that shows how people move from one stage to the next within your sales cycle. It is specifically changed with the flow step, Change Revenue Stage.
The Lifecycle is the practical application of that reporting layer. Theoretically you can have the exact same stages for each model. Or they could be slightly different. Your revenue model could be cleaner, simplier. Your lifecycle model could be more complex. This progression is by smart campaigns & Change Data Value. Just like any other way of changing data.
Make more sense? Or just as confused?