When you think of clouds, you probably think about the white things that float above your head. I don't. I think about the data centers that host the storage for my files online. Google, Microsoft, Apple and many others like DropBox all own massive server farms to provide customers like you and me, a private and safe locker to backup all of your data so it is safe for ever. That means that you will never have to worry about your girlfriend scratching your favorite CD, or worry about your computer crashing while you were typing your final thesis. It is also very useful to transfer files to your phone without having to use any wire.
Once again I will be honest, before this, I did not know anything about SkyDrive. I knew it existed but I was not interested since I do not own a Windows Live ID and I do plan to get one any time soon. But I have to be objective, it kinda looks nice. It's all integrated with the Hotmail interface we are all use to, you can access it from anywhere just like the others and it lets you share and collaborate on documents with other users like Google Drive. That means that you can have more than one person editing the document at once. Very cool feature. Also, you get 7 GB of free storage when you sign up.
No surprise, I use this one and it is my favorite. Just like Microsoft's, you can share documents and access it from anywhere. The main difference is that with Drive, all your files are categorized. Your files, pictures and documents will be in Google Drive while your videos will be uploaded to your YouTube account and your music will be in Google Music. In summary, Google gives you virtually unlimited space to store your videos, room for 20,000 songs, 5 GB for random files and documents and that's without counting their old picture service, Picasa. At last, I thing the biggest point in Google cloud's favor is that Google Music is available over the Web, so you always have your music library, wherever you are.
Ob-to-the-viously, you can't use Apple's cloud service if you don't own an Apple device and still, even if you own an iDevice, it needs to run iOS5 or the latest version of their OS for Macs. They give you 5 GB of storage for your files but anything you buy on iTunes will be available in the clouds. The iApplications data is unlimited and you can upload up to 1000 pictures every months. My friend who's an Apple fan-boy told me he thinks iCloud is good for contacts, calendar, email and app data but he does not really use it as storage space.
For me, DropBox was the first cloud. It is also my favorite of the small ones. DropBox is an online storage service that will offer you 2 GB to start with and 500 extra megabytes each time you invite someone and they join DropBox - very nice business model. I use it all the time to transfer files to my phone but it can be used for many things. For example, my friend has to do work for school and he was teamed up with random people. Since everyone uses a different cloud service, they created a new DropBox account and shared the password with the group so they can all use it for their assignment. Brilliant !