a boot animation is a bunch of images placed (not compressed) in a zip file along with a text file that tells Android how to animate the images you included.
So, to convert a video to a boot animation, you need to grab individual frames of the video and convert it to a specific format (PNG 8 or 24bit).
If you want to look at your boot animation file, go to
/system/customize/resource
and look for a file called bootanimation.zip. In some ROMs, an mp3 file will also reside in this location, which is the sound that plays along the boot animation. The sound file is optional.
Inside the .zip file is a desc.txt file. Here's a sample from an Evo and what the values mean:
480 = width
800 = height
7p = 7 FPS
0 = times to loop animation. 0 = loop all images in zip forever
0 = pause between loop. 0 = no pause. 10 = longest pause
android = folder containing images
The images are animated in filename alphabetical order. So name your images accordingly.
I believe Mr. Ed in the Evo 4G forums created a good tutorial for creating your own bootanimation. What I presented here is the bare essentials; there are a few gotchas you're likely to run into and can stump you for a while