Cloud is essentially a platform or service you can pay for to use instead of having to manage servers yourself.

That's basically it.

This platform however comes with some restrictions and possibilities. For one, your entire application architecture has to change and be reworked for you to be able to put your applications and services in the cloud. You have to learn about the roles and restriction and handle a lot more of parallel execution so that if you lets say have a queue of transactions to be processed and you have several worker roles you have to make sure these workers does not do each others jobs etc.