Think like a journalist (in general, not photo-journalist). "Localize" your speech - relate it to what they know, then work backwards.

Many digital shooters will take their files to a store to have them printed RA-4. You could explain it works just as it had before digital (as far as automated 1-hour photo services). Of course now the negatives are scanned, and exposed to the paper differently. Start with how it's done now, and then explain how it was done "optically" by a minilab. Not a huge difference. Describe the darkroom as simply a manual version that is far more versatile. As we get more "modern" we get more automated, and arguably more convenient. Leave quality out. I like to debate quality, but that would be a subject for _after_ an informative speech.

Then use the Photoshop analogies people have mentioned above to show 1) where the computer tools got their names and 2) how the same things were done before computers as we know them. Perhaps a tiny sidebar about how the amount of light (duration of exposure, aperture, etc.) affects the print, and how computer tools basically just emulate this.
Explain how this is similar for film, paper, and even a digital sensor. Not every digicam has the same sensor; some sensors react differently, just as papers can be different.
Same concepts, different methods/materials.

Personally, I don't like digital, but it has it's place. Different tools for different needs, and there is nothing wrong with that.
It is also very convenient.
Give digital credit for making things convenient for people who would otherwise not be able to do employ photographic processes. Not only does this relate to darkroom versus digital, but expensive versus cheap digital. One can get a cheaper digital camera and The Gimp, Free Open Source Software (FOSS) that is a powerful Photoshop equivalent for free:
Before digital, it was not as easy for just anyone to do more than take snapshots and settle for what a minilab gave them.
People can now also get a cheap point-and-shoot, have the film scanned when developed, and then do "darkroom" work without a darkroom. There are disadvantages too, but you are trying to inform people about the concept, not which side of the bread should be buttered.

Also keep in mind there are other, more general, analogues that may relate to some of your audience. For example, most people take their cars to a garage for repair - they don't need to know how to fix anything. However, some people like to do some of there own work. Fixing my own car is cheaper, but more importantly, I really enjoy it. The same can be said with landscaping yards, baking cakes and pies, etc.
I can buy a lousy pie from one store, a good one from another, and I can make my own. Each has it's advantage.

As for most things in life, focusing on differences can pull people apart. Differences are important, but focusing first on similarities helps draw us together.