Description taken from Arcade version of this game

In Frogger you are a frog perched on a sidewalk, trying to make it to your home bay. Between you and this place of sanctuary lies a busy highway and a perilous river. To make it across the highway, you must hop between cars and trucks, frantically dodging these vehicles which move at varying speeds. Once you have crossed the highway, you are safe on the riverbank (unless a snake bites you).

To get across the river, you must hop on alligators, turtles, and floating logs, timing your jumps so you don't land in the water, on an otter, on a snake, or in an alligator's mouth. You must also be careful not to float off-screen. Sometimes a lady frog will appear on a log. If you hop on her, you will receive extra points.

Once you are floating on the far side of the river, you should then hop Frogger into one of the five home bays. If an alligator head appears in a bay, you shouldn't jump until you are lined up with the next available bay; if a fly appears in a bay, you can gobble it down for extra points. When all five slots are full, the action starts at the beginning, with you back on the sidewalk and all of the bays empty. ~ Brett Alan Weiss, All Game Guide