Blender - Rendering from the command line

Render an Image:
blender -b [directory of file] -o [output] -f [frame number]

Render a Video:
 blender -b [directory of file] -o [output] -s [start frame] -e [end frame] -a

http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Command_Line

Creating a video using a sequence of images and FFMPEG

I've been studying Blender for a while now and have developed a preference of saving the animations as a PNG image sequence instead of video files. Later I can load them in Blender's VSE, o kdenlive, and edit the video.

But sometimes I would rather just see how the animation is going without having to load the files again and render them into a video file.

 ffmpeg -f image2 -i %04d.png animation.mp4