What is the difference between and in RSS?

Question

I have the feeling, in every RSS.xml file, both the pubDate and the lastBuildDate match.

I am sure that this one, is not always true...

So firstly, what is

Answer 1

While the other answers here do provide some good information, I feel the need to elaborate just a little bit for any future visitors.

pubDate
The publication date for the content in the channel. For example, the New York Times publishes on a daily basis, the publication date flips once every 24 hours. That's when the pubDate of the channel changes.

lastBuildDate
The last time the content of the channel changed.

So, taking the New York Times as an example again, the is the date the feed was published while the would be the date the content inside the feed changed. In the end, I would view the as the date the feed is published and the as the date any content in the feed was last modified.