In Flash two types of tweening is possible - Motion tweening and Shape tweening.

Motion Tweening: Motion tweening is used to tween the changes in the properties of instances and other objects. With motion tweening we can change the position, size, rotation, and skew objects to create motion from one frame to another frame. Additionally, we can also tween the color of instances, make the color to fade in or fade out. To tween the color of any object we must convert them into symbols. See Creating symbols for that. 

We can create motion tween by using the steps below:

1. To create a new file click on 
                          File > New
    A layer will already be there with a blank frame. First name the layer 
    as "Motion Tween". Select the layer by clicking on layer name.

2. Now draw a object by selecting a tool from the toolbox or drag and 
    drop an instance of a symbol that is already in the library on the stage. 
    Now the object will also be in the first frame of  the layer. Don't forget 
    to convert the object into symbol if it is not a symbol already. 

3. Insert a frame on the layer, upto which point you want the animation to 
    last, by clicking Insert and selecting Frame.
                         Insert > Frame 
    or by right clicking ( for Windows user ) on the point of interest and 
    selecting Insert Frame.

4. Select the first frame of the layer. Click on Insert and select Create 
    Motion Tween
                  Insert > Create Motion Tween
    or right click ( for Windows user ) on the frame and select Create 
    Motion Tween.

5. Click on last frame and insert keyframe by clicking on Insert and 
     selecting Keyframe.
                        Insert > Keyframe
    or right click ( for Windows user ) on the frame and select Insert 
     Keyframe. Now the layer should look like this. 

    In this example the animation lasts for 20 frames.

6.  Select the first frame and set the properties of the object the way you
     want the object displayed at the begining of the animation. After that 
     select the last frame and make changes in the properties of the object
     for its display at the end of the animation.

The properties that can be changed to create an animation are:

1.  Position:  You can change the position of the object at a particular 
     keyframe just by repositioning the object on the stage at that 
     particular keyframe. Select the keyframe and drag the object to the 
     desired position.

2.  Scale or Rotate: You can change the size of the object in the 
     particular keyframe to create zoom in or zoom out kind of effects.
     To change the size go to Modify select Transform and click Scale.

                              Modify > Transform > Scale

    You can also make the object rotate by using the rotate option. Select 
    the keyframe in which you want the object to rotate. Go to Modify, 
    select Transform and click Rotate.

                              Modify > Transform > Rotate.

    Windows user can right click on the object in the particular keyframe 
    and can select the Scale or Rotate option.
 

Right click Scale option	Right click Rotate option

3. Color Effects: You can create color effects on the objects in Flash and 
    you can use these color options to create color tween. Make sure 
    before using the effect options that the object is a symbol. To use 
    these options go to Windows, select Panels and click on Effects. The 
    window will be as as shown below.

   You can use five effect options Brightness, Tint, Alpha or Advanced 
   option. Select the keyframe on which you want  the color property of 
   the object to be changed. Now open the effect window and select one 
   of the option. You  will get few more options on the window and by 
   changing them you can get the desired effect. Suppose you chose the 
   Alpha effect.You can see the change in the window as shown below 
   the effect window. You can change the percentage value and can get 
   the desired effect. The same can be done with Brightness. With the 
   help of Tint  we can change the color. With the Advanced option we 
   can change color as well as Alpha. 
 

Motion Tweening along guided path: We can create motion tweening along a user define path.For defining the path along with you want the object to move follow the steps written below.

1. Do first three steps as we have done while we were creating simple 
    motion tweening.

2. Now insert a Guide Layer. Guide Layer is a type of layer, where you 
    can draw a desired path for your object to move along. The beauty of 
    the Guide Layer is, it will not be visible in the actual movie. We can 
    see it on the stage while we are creating  the movie. To insert the 
    Guide Layer click on the icon   at the bottom left corner of the 
    timeline window. Alternetive way is go to Insert and click Motion Guide.

                             Insert > Motion Guide

3. While the Guide Layer selected, draw a desired path on the stage 
    using Pencil   tool or Line   tool from the Tool Box.

4. Select the layer of actual object and create motion tween using step 4
    and step 5 of motion tweening. 

5. Place the object at the starting point of the path in first frame and at 
    the ending point of the path in last frame. Now you have created the 
    motion along desired path.

Shape Tweening: Shape tweening is used to tween the shape of  instances and other objects.  With shape  tweening we can create the effect of transforming one object into another object. Due to tweening we just need to define the object in first and last frame of the animation. We do not need to worry about the intermediate layers. 

You can not shape tween symbols and text objects. To shape tween a symbol or a text object , you need to break apart them. You can break apart the the boject by selecting the object and go 
 
                                          Modify > Break Apart    

It is better to do shape tweening on simple objects that can be created by using tools in the Tool Box. You can try shape tweening on symbols and text objects when you get comfortable with creating tweening and other concepts of the Flash.

We can create shape tween by using the steps below:

1. Draw the object you want to transform into other object on the stage 
    using tools in the Tool Box. It will get place in the first frame of the 
    current layer.

2. Insert a blank Keyframe on the layer, upto the point you want the
    animation to last, by clicking Insert and selecting Blank KeyFrame.
 
                            Insert > Blank KeyFrame 

3. Select this blank keyframe and draw the second object in which you 
    want the first object to be transformed

4. Now go to Windows, select Panels and then select Frame. A window
    will open as shown below. 

                         Windows > Panels > Frame

4. Select the first frame and then select Shape in the Tweening option. 
    The Shape Tweening is done. The timeline will look like as shown 
     below.