What is Cloud storage?
Cloud storage is a service, where you can buy storage space for your data. Usually, your data are stored on multiple servers owned by the storage provider. First of all, it is huge business in today world but probably the biggest problem of cloud services is, that all your data are in responsibility by some company.
It is very dangerous, that someone can access to your sensitive data, because your data are in one place, hosted by a company. Next negative factor is price, which is driven by big companies like Apple, Microsoft or dropbox. Everything in this business stands on the trust between you and company. If you want to know technical detail about cloud storage, you can check Wikipedia article.