In email analytics, the viewed statistic refers to all your contacts that opened your email. If a reader viewed your email, and also clicked a link or shared your email, you'll find the contact in your Engaged stats and as well as your Viewed stats.
Campaigns are tracked using a 1 pixel by 1 pixel image-we call this a tracking beacon. The tracking beacon is an invisible image in all campaigns that, once downloaded, lets Email Marketing know your contact viewed the email. Before your reader opens the email and allows images, or clicks a link, we have no way of tracking the recipient behavior. So once your reader downloads the images in your campaign, we call this a "view". If your reader received the campaign successfully, but has not viewed images, or clicked a link, then we call that accepted.