A turtle in LOGO and other turtle graphics systems is a way of describing geometric shapes using a string. I'm not explaining this well. Let me try again, this time with an example. In LOGO, the turtle is usually depicted by a isosceles triangle which begins at the centre of the screen, pointing to the top. If you give a command to move forward by a set distance, the turtle moves up the screen, drawing a line behind it. The turtle can be told to turn right or left, to move backwards, and to change the colour of the line drawn, the thickness, and even to stop drawing for a while. In some systems, the state of the turtle can be pushed onto a stack, allowing for branched systems to be drawn. Extensions of this model to three-dimensions combined with the use of L-systems, and good photo-realistic rendering can produce convincing images of plants.