Until recently, sharing files on your Google Drive account was generally done by opening Drive and starting an email (or by copying a link). Now, you don’t have to jump into Drive at all. So long as you’re using the new version of Gmail’s Compose Message tool (that would be the little message in the bottom-right corner of your Gmail window), you can link to files right from the message. If you haven’t already opted into this new feature, you’ll need to in order to enjoy the Gmail/Drive integration.
The new, opt-in Gmail Compose tool integrates with Google Drive.
Before your message goes out the door, Gmail checks to see which sharing options you’ve selected (such as Everyone, or Anyone With A Link) and may prompt you to double-check your permissions selection before you send the email out the door. That should help you avoid accidentally sharing files with more people than you meant to.