Scalability is the provision of expanding or reducing the extent of services you require, depending on the situation. This means that if you require more of a service, you only have to ask your service provider and he will provide you with that additional service for an additional charge.
Technically speaking, all applications can be run by a cloud but it is always a good idea not to put system dedicated applications (that run on minimum latency) on the cloud. Good examples include Windows default Disk Defragmentation Tool.