Layers : Layers are just like transparent sheets which can be placed on top of each other and we can paint on them.Whatever we want to create on the stage we need to place it on the layer. We can add layers, delete layers and change the position of the layers.The layer we are currently working on is called the active layer.We can name the layer by double clicking on the layer name. 

We can insert a new layer by clicking on the plus   at the bottom left corner of the timeline window or by go to
                                              Insert > Layer

Frame: Layers are divided into small parts called a frame .The time of the animation depends on the number of frames and the frame rate. 

Keyframe : When we need to make any change in the animation we need to insert a keyframe on the layer. We can insert keyframe just by right clicking on the frame,where we need to insert keyframe, and clicking on Insert Keyframe. Alternative way to insert keyframe is by clicking on 
                                       Insert > Keyframe

By Insert Keyframe	By right clicking

Frame Rate : Framerate is measured in frames per second. We can adjust timings of animation by setting the frame rate and number of frames. For example if we have our animation in 20 frames and we set the frame rate as 10 frames per second,  then the animation will last for 2 seconds. We will see later how to set the frame rate when we work on setting the movie properties.

Symbols and Instances :Symbols are reusable graphics that we create with the drawing tools in the toolbox. When we place a symbol on the stage or inside another symbol, we  create an instance of that symbol. Symbols can be of three types-- graphics,button or movie clip. We can create a symbol by clicking on 
                                                 Insert > New Symbol

We will learn to create symbols later when we are creating a simple movie. 

Library: The Library is a place where all the objects we import from outside of Flash and all the objects we create as a symbol, get stored. We can reuse them whenever we need them. To open the library window go to 
                                           Window  >  Library

The window is as shown below. We can drag and drop the objects from the library on the stage whenever needed.