A written explanation
Right now I'm looking at Mozilla. I've got a left arrow (with a small triangle pointing down), a shaded-out(?) right arrow (no small triangles), a curved upwards arrow, and a shaded-out X. Thanks to the miracle of tooltips, by keeping my mouse on them for an inordinately long amount of time I can get an explanation of what they are. And in English (not Ancient Egyptian), to boot!
On the bottom I've got half of a ship's wheel, a sealed envelope, and two cryptic images that I can't even name. Alas, no
Rosetta Stonetooltips here, so I've no idea what they are or how to read them.
The original idea of icons were that they're intuitive and needed no explanation. Unlike textual items. Oh well.