Individual Email: As soon as something occurs in a given category, like Introductions, Invitations, or Job Notifications, LinkedIn sends you an e-mail with that one item in the e-mail.
Daily Digest Email or Weekly Digest Email: Instead of individual e-mails, LinkedIn groups all the activities in a given category and sends you one e-mail in a digest format, with a summary at the top of the e-mail and the detailed activities below the summary. Note that Daily Digests are for Group updates, while Weekly Digests are available for General and Group updates.
No Email: Turn off e-mail notification. You can read the message, though, when logged in to the LinkedIn site.