In response to a question posted in a comments section elsewhere, QC99.tsilvius crafted this elegant response. Since this question comes up so frequently, I'm reproducing Todd's advice to preserve it for posterity.
Plan A. Creating an Announcement that your students can get in their email.
Click Announcements in your class, on the class menu, at left.
Click the "+ Announcement" button, top-right (pictured at right)
Type your message, include links, etc and click Save button at the bottom.
Additionally, you can attach a file or schedule your announcement to send off to your students (even if you're not logged into Canvas) with the delay posting feature. Click here to learn about the Announcement tool from this Canvas Community help doc.
The only "downfall" of this system is that every user has the freedom to turn-off their Announcements in their account Notifications. This means if your students have done this, they'll never get that course Announcement in their email.
Plan B. "Email an Announcement to all of my students"
Be sure your Canvas account Notifications are set for you to receive Announcement created by you (see pic at right).
Create and Save your course Announcement using the directions and doc above.
When you receive the Announcement you made in youremail, use the forward option in email to send to all of your students. Students with access to your course will be able to access links and attachments in the Announcement email that you forward to them.
If you know that students have clicked the "X" (Do not send me anything) option for Announcements in their Notifications and want to insure that your Announcement communication reaches their email inbox you can use this approach. This is also helpful just in caseyour students did not set up their Canvas profiles with a primary email address and you know they indeed do have an email address.
For example, our SIS integration with Canvas does not providestudent email addresses because student email addresses are not available in our school district's SIS. IOW, student emails do not automatically populate in student (auto-created) Canvas accounts.
Plan C. Using the Canvas Internal Messaging for your announcement.
You can also copy/paste the email you get from Plan B above into a new (Conversation) message sent using the internal Canvas messaging system.
If the students' default Notifications are untouched for Conversations they should receive an email that they got a message.
If they don't own an email account their Canvas "Inbox" icon will display a new message indicator in their global navigation icons. (see pic at right)
Whenever you have questions about the Instructure Community (how things work in this forum? need to change your username? stuff like that), post them to the Community Hub and a member of the Community Team will assist.