Using this tutorial, you will learn how to create a realistic smoke effect.

Step 1.
Create 400X400 pixels document. Then apply Edit > Fill...(black). Choose the Brush Tool and draw a white line.

Step 2.
Now we need to distort this line. Apply Filter > Liquify and distort this line to create a smoke. Then apply Filter > Blur > Blur.

To get the same result you may use Smudge Tool. And that's it.