Using layers

Layers make it easier to work with very complex drawings.  They allow you to separate elements of your work and change things you want to change without affecting other parts of your drawing. 
All Illustrator documents contain a single layer to begin with.  You have the ability to add more layers to your document by clicking on the arrow in the upper right hand corner of the layers window and clicking New Layer…  Each new layer you create will draw paths in a different color.  Select the color you wish to use and click OK.  To add an object to a layer, click on the name of the layer you wish to modify and draw. 
You may hide layers that you’re not working with by clicking on the eye to the left of the layer.  The images in that layer will disappear.  To bring them back, click on the place where the eye should be. 
It is also possible to expand a layer to see the individual objects that it contains.  To do this, click on the arrow to the left of the layer you wish to expand.  The contents of the layer will be displayed below it. 
You may also lock layers so that they cannot be edited.  Do this by clicking on the lock to the left of the name of the layer you wish to lock.  Now you will be unable to make any accidental changes to that layer