You can share individual files, or you can share an entire folder.
Files can only be shared by using a link to the file. You can email the link, or get a link to a file or folder and use it however you want to, like in an instant message or a document. Sending a link is useful if you need to give access to people on an as-needed basis. The only way the recipient can access the file or folder is by using the link.
You can also add members to a folder, giving those people access to all the content in the folder. This is useful when you're working on a large project, for example, and people may need to continually access information.
You can even add external users who may not be a part of your organization to a folder, provided your service administrator allows it and their email address is registered with your service. External users can collaborate on objects to which they're given access, but they cannot be assigned the manager role. This safely limits their ability to create and remove content.
See Share Files and Folders for more details.