I just read the following article on Google +.
New toolbar button setting: icons or words
When we launched Gmail’s new look, we did something different:
we put icons on the buttons. Icons brought consistency across
languages and solved problems with functions that had long
names.
Some people loved the new icons. Others, especially low vision
users, found words easier to distinguish. We’ve been listening
to your feedback and we now have a setting that lets you turn
the icons into words. You can make the change by clicking the
gear to open Settings, and changing your button labels on the
General tab.
Love the icons? No problem. Icons will continue to be the
default, and you can always hover over the button to see its
name.