This picture shows the border between the Mare Serenitatis (the Sea of Serenity, to the upper-left of the picture) and Mare Tranquility (the well-known Sea of Tranquility, to the lower right of the picture). The different shades of grey show the different lavas that flowed into these areas. In addition to the well-defined border between the two Mare there are some other patches such as the areas around craters Dawes and Jansen.
Relative ages of craters in this image can be deduced by their relationship with the mare lavas, for instance Plinius seems to have excavated the lava and despite an apparent breach in its northern crater wall, has a rough, rather than lava-covered floor. Plinius therefore clearly appeared after the lava, whereas Vitruvius appears to have a lava-covered floor, and Jansen has almost completely filled, making the crater rather indistinct, so those craters must have been formed before the lava flowed.
Move the mouse over the image to see feature names, move it out to see the undecorated picture.