CTR (click through rate) and CTOR (click to open rate) are two KPIs (key performance indicators) or metrics that are used in email marketing to gauge the performance of an email campaign. If you are using a consumer email marketing services sending platform such as MailChimp, Constant Contact, Aweber, etc., they may not display CTOR. You would need to calculate it separately by taking your Unique Clicks divided by Opens.
CTR gives an indication of how your overall email campaigns are doing in the broad sense but can be distorted since there are technical reasons why this data isn’t accurate such as emails going to Spam, Bulk or sent to the Promotions tab in Gmail or just poor subject lines. CTOR gives a good indication of the quality and effectiveness of your email body content.