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.
See Sharing Your Content and Sites with Others for more details.