With Drive, Google is essentially making a play for the same territory currently occupied by services like Dropbox and Apple's iCloud— It's a way for you to sync and share your data and documents across multiple computers and devices. But since this is Google, Drive has the added benefit of being synced across all your other Google accounts. Any Word document you save in Drive, for instance, can be easily shared an edited via Google Docs.
Google Drive is available for both PC and Mac (though the mobile app is only on Android, currently) and it can open over 30 different file types right from the Web, including various video formats and Photoshop files.
The service is free for up to 5 GB. If you want more than that, you can get 25 GB for $2.49/month; 100 GB for $4.99/month or 1TB for $49.99/month.
All of which is to say competition just got very stiff for Dropbox, which offers just 2 GB of free storage and, despite an impressive 45 million users, will have to contend with Google's massive captive audience.