If you're subscribed to receive email from certain senders, the messages you receive from them will be enhanced with an interactive gadget that has up-to-date content from their website (you'll also see an icon in your inbox identifying these messages).
For example, if you receive a Pregnancy Bulletin newsletter from Babycenter, you'll be able to view up-to-date content, including the baby name of the day, and browse though the current top 100 baby names within the message. Aside from the convenience of being able to interact with certain websites from inside Gmail, the branded content will help identify that your messages are legitimate and not spoofed (we'll only show branded content when the sender authenticates their mail). We're currently testing this with a small number of senders and will decide whether to make it widely available based on user and partner feedback.
If you'd like to stop receiving enhanced messages from a particular sender, just open a message and click the down-arrow in the upper-right corner of the message and select Do not customize mail from [sender]. Any messages you receive from that sender in the future won't have images displayed by default, or any interactive content within the messages.