Push media is the term used to describe media that is provided to the end user when when it's convenient for the provider. Ads on a webpage are a good example of push media - the user doesn't do anything, they just appear - "pushed" down the pipe to them.
Pull media is just the opposite: media that is delivered only when requested by the end user. Most email configurations are a good example of pull email - you fire up your email client which is a request for it to go the server and get your mail.
These terms were easy to talk about 5 years ago because of the limited ways in which the technologies were being used. But how, perhaps it's not so easy to differentiate. Podcatching, for example, is a hybrid.
Podcatching requires the user to subscribe to a podcast feed which is a user request and therefore falls into the category of pull media. However, once subscribed to the feed, the content is provided as it becomes available without any further action on the end users part. Which cleanly falls into the category of push media.
So which is it? Is podcatching pull media because the entire process has to start with a user request? Or is it push because the content is delivered when available? Or do we need yet another new word?