Learn has a useful Announcements tool which allows you to post messages to students enrolled on your course. Announcements remain on the course site (unless you delete them) and they also appear in the Learn applet on MyEd. When creating an announcement, you can select to send an email to all students with the same message.
Notifications alert the user to activity on the course site. This may include new content being added or due dates for assignments. The default for notifications is a daily digest sent out after 5pm, however, these can be configured by each user so don’t assume a student has received a particular notification.