Personal Level Indicators
How can I tell if a message was sent to just me or to a mailing list?
Gmail displays arrows, or 'personal level indicators,' next to messages in your inbox so you can tell if a message was addressed to you, a group, or a mailing list. A single arrow (>) appears next to a message when it is sent to you, and a group of others. Double arrows (>>) indicate that a message is addressed only to you. Arrows won't appear next to messages sent to a mailing list.
How to enable personal level indicators in Gmail - Video |