What is the difference between Bitmap and Drawable in Android?

A Bitmap is a representation of a bitmap image (something like java.awt.Image). A Drawable is an abstraction of "something that can be drawn". It could be a Bitmap (wrapped up as a BitmapDrawable), but it could also be a solid color, a collection of other Drawable objects, or any number of other structures.

Most of the Android UI framework likes to work with Drawable objects, not Bitmap objects. A View can accept any Drawable as a background. An ImageView can display a foreground Drawable. Images stored as resources are loaded as Drawable objects.

Bitmap is not an image. Bitmap is a map of bit (note name: Bit-map). And this map represents pixels on which you can draw something. It may be your own custom bitmap (not image), for example square:

Bitmap bitmap = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888);

or you can create Bitmap object from image:

Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.your_image);    

A Bitmap is a pixels holder. And Canvas is used to draw something on your bitmap (on Bitmap pixels).

Everything about Drawable is well described above.

TL;DR

Some people write that you draw on Canvas. You don't draw on Canvas. You draw on Bitmap pixels with Canvas helper method.

Bitmap bitmap = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap);
canvas.drawColor(Color.RED) // now all bitmap pixels became red