You can click on the Help sign at the main menu for an onscreen tutorial.
Gameplay is rather simple. You are the snake on the screen. All critters on-screen are fair game, including you!
You can bite any other snake in any segment besides its head. If you're longer than a snake, then you can eat it head on. When you're longer than another snake, you'll be a bit darker than that snake. If you're shorter, then you'll be a bit lighter. For every snake you eat head on, you gain a segment in length.
The other critters, such as the frog and bird, are your sustenance. For every one you eat, you grow a segment.
Occasionally a power up will appear. They might make you invincible, superfast, able to go through the walls of the maze, or grow to your maximum length.
At some point, you may lay an egg, if the egg is still around at the end of the level, you get a bonus life. But be careful, when you lay an egg, you lose a segment. If you're only one segment long, lay an egg, and that snake will perish.
Of course, the other snakes are just as hungry as you are, so watch out, and avoid being eaten.