You do not have to have a Windows Phone to use Skydrive, but this feature is now built into Windows Phone 7.5. It is simply a type of cloud storage available; much like Dropbox, Skydrive allows you to store, organize, and download your files, photos, and favorites on Windows Live servers and then access them from your Windows Phone.
The SkyDrive storage meter shows how much storage space you've used and you can arrange your files in folders and subfolders. You can even choose permissions for each top-level folder you create, allowing others to have access to your files. Currently, you get 5GB of free storage when you sign up with Skydrive.