Alexander Musarra writes ...
Let me first generalize pop music. Popular music generally focuses on having a good, danceable groove, and a simple, melodic hook. Popular music is also relatively short, 3 minutes being average. Harmonically they are generally diatonic (within the major scale) with little, if any modulations (change of key).
Progressive rock, on the other hand, explores these different elements in more depth. Rather than a danceable beat, progressive rockers may experiment with odd time or unusual and interesting rhythm patterns. They are often inclined to make melodies more complex and unconventional, rather than lyrical. Their songs forms are usually very long, with many different sections and interludes. Progressive rock is harmonically adventurous, with lots of chromaticism and modulations.
Oh, and there are also lyrics to consider. Pop music usually deals with love and romantic relationships. Progressive rock is usually more conceptual or with longer, original narratives.
Progressive rock goes into the bundle with classical music and jazz. It is not always "pleasant" sounding or accessible. It is arguably more challenging, dynamic, and artistic. I think of it as like the difference between American cheese and bleu cheese.