Brand new SkyDrive users get7 GB of storage for free. Existing users (those with accounts as of April 22) continue to get up to 25 GB of storage for free if they opt to do so. After that, an additional 20 GB costs $10 per year; an additional 50 GB costs $25 per year; and an additional 100 GB, $50 per year.
With the SkyDrive app for Windows, everything put in a SkyDrive folder on a Windows PC is automatically kept in sync between PCs, Macs and SkyDrive.com, according to the download site. Also on the feature list:
Ability to access SkyDrive directly from Windows Explorer — photos, documents, and other files
Option to add new files to SkyDrive by dragging them to the SkyDrive folder
Ability to organize files and folders in SkyDrive, just like any other folder
Option to connect back to the PC if the user forgets to put something in SkyDrive
Update: Microsoft also updated the iPhone and iPad versions of SkyDrive today, as well. (Thanks for the info, @tomarbuthnot.) These updates support the new Retina Display, and add a couple other new features.