Cloud storage is simply a way of storing your information somewhere on the internet (usually a server belonging to a company)
Many computers and phones now come with cloud storage, as do many email accounts, so you may not even be aware that you have access to them. There is some speculation that Cloud Storage is the future of computing, and that we will have devices that access the cloud, rather than storage of data.
Why are they useful?
1) If you use a cloud server, and synchronize your device with it, it will automatically back up all your files for you. This means you won't ever run the risk of losing your data. (Free cloud servers do have size limits, so if you have lots of big files, you may need to use more than one, or buy storage space) Those of you old enough to remember how easy it was to lose all your data when computers first became available, will appreciate this service.
2) Many cloud servers also include word processing software, so you don't have to have Microsoft Office on your computer to create or read Office files.
3) Document sharing: if you have a document(s) you want to send to someone, an easy way is to store the document(s) on a cloud server, and send a link to the document. This avoids issues with the size of attachments to emails.
4) Collaborative working: Google Drive is particularly good for this, as multiple people can be reading and working on a document at the same time. You can share a folder with your team, and everything related to the project can be uploaded here, so everyone has access to it instantly. There are no issues with version control, as long as everyone works on the online document.
5) No software maintenance: the company providing the cloud storage keeps the software up to date, and everything working smoothly. No more issues with your computer not working.
6) Security: Cloud service providers invest a significant amount of money in ensuring your data is safe on their server. The caveats are: - ensure you have a secure password, and don't give it to anyone. Be aware that the your data may not be private from the cloud storage provider (Google say they have access to anything on their servers - this is one of the reasons I hate Google) Antivirus protection is usually built in to the system.
Finally, if anything is really sensitive, don't put it on the internet, or any device connected to the internet!
7) Access your files from anywhere. No more memory sticks, floppy disks or taking your computer anywhere. If your documents are on the Cloud, then you can use any machine with an internet connection to open them.
Which one should I use?
To some extent, this depends on what you want to do, and whether you want to spend any money. I tend to have multiple accounts, and use them for different purposes - a way of compartmentalizing the different freelance jobs I do. All of the ones listed below are free for personal use (prices vary for business use), and I use all of them. Three are others out there, but I will only recommend services I use, or have used.
To me this wins hands-down for functionality - the reason I love Google. If you have a Gmail email account, you have access to Google Drive:
A Google account also comes with it's own email account, calendar, and a range of tools. Google Hangouts is a way of having a video conversation with someone on the internet.
This is Microsoft's version of a cloud server. It includes an email account, document storage, Skype (Microsoft's version of video conversations)
Box doesn't have as much functionality as either Drive or OneDrive, and instead focuses on doing one thing well - document storage and sharing. We currently use Box for managing shared documents on a research project. You will need to link it to your computer software or to Google to open documents.
Like box, Dropbox provides secure online storage of documents, and focuses on document sharing. There are a range of pricing models, starting with a free basic account. If you want more storage, then you would need to pay.
You can find a really good blog here which talks about others, and gives an evaluation of all of the different services.
If you don't have an account, I'd suggest starting with Google, because most sites link to it. But, be aware of their approach to privacy!