Summary - Read

Unique opens with images	The number of recipients that opened the message. They may have opened it multiple times. They were tracked as having opened the message because they downloaded images.
Unique opens without images	An educated, conservative estimate of those that read the message in txt format or HTML format without images. The estimate is based on having data on those that clicked, but were not tracked as opens.
Readers (or Overall Unique Opens)	The number of recipients that opened the message. They may have opened it multiple times. They were tracked as having opened the message because they downloaded images and/or clicked on a link.
Opens with images	The number of times the message was opened. The message was tracked as having been opened because images were downloaded. Recipients can open the same message multiple times. So Opens is always larger than Unique Opens.
Opens only txt	A conservative estimate of the number of times the message was opened in text format or in HTML format without downloading images
Overall opens	The sum of all Opens.