Render with transparency for video

I was wondering how you can render an object with a transparent background in a video. This is for pasting on top of regular video for special effects.

You need to use a codec that can handle transparency. Either something like a .png image sequence or a video codec such as the Quicktime Animation codec that can handle transparency.

Thanks for the response, but how would I go about doing that, exactly? I know how to make a PNG sequence, but I’m not sure how to use specific codecs outside of the ones in the menu when the output format toolbar is opened.

Sorry if I sound woefully underprepared; I just never had to do something like this.

The codecs you have available to blender are in the Render Output settings, just select the format such as .png/quicktime etc. If you select quicktime additional panel should then appear for you to set what codec you want and additional output settings. For the quicktime codecs you may have to install quicktime (I’m using a Mac so if you use windows/Linux I’m not sure what you have by default)

Which codecs, specifically, should I be looking for?

Use png or tga image sequences.
One does not use video files to composite unless there’s no other way, especially not compressed ones. That’s extremely bad practice.

You’d be ending up compositing compressed lossy frames over a most likely ocmpressed lossy frame source, compressing it again on export.
Try to stay as uncompressed and lossless as possible until the final export to a video container with compression.

For compositing you also got the option to use premultiplied alpha or straight alpha, it depends on the project on what you use.

you can also use the larger (more industry preferred) openEXR format for your sequences.
you can read more about it here: http://www.openexr.com/

but in a more direct answer to your question, you would probably use the node editor , and overlay it ontop of your movie input and hit “render” or link the composite tree to a file output
you can learn more about the node editor here:
http://www.blender.org/features-gallery/feature-videos/?video=imaging