Trello Icons – What Does the Little Pen Icon Mean?

If you hover your mouse over the little boxes, you'll see the following tooltips:

Blue: "This member is an owner of this board" (or organization)

Green: "This member is online and viewing Trello"
Yellow: "This member is idle. They have Trello open but are not looking at it"
Gray: "This member is offline"

So in the image above, top left user is active. Top right users is an owner, and is idle (or away). And the two users on the bottom are board members who are not currently logged on to Trello (inactive).

The icon for the activity stream is the same icon as the one in the drop down, so the number represents new entries in the activity stream. Would suggest adding "Activity" to the Glossary in the help section.