Some heavy metal subgenres have pretty straight forward songs. With other genres you never quite know what you're gonna get. With progressive metal you don't quite know what you're gonna get except it will most likely not be very straight forward. Confused? Read on and listen!
Progressive metal can most easily be described as a mix of the progressive rock from the 70's and heavy metal from the 80's. The genre takes the long songs, odd signatures, surprising turns and intricate playing from progressive rock and adds the distortion and heavyness of metal.
One of the first bands that mixed elements of progressive rock and hard rock in the 70's was Canadian band Rush but progressive metal in itself didn't become a distinct genre until the second part of the 80's with bands like Queensrÿche, Fates Warning and Savatage. These bands all started out playing fairly traditional metal but soon began to experiment more. The music got more intricate and epic and they all added a lot of surprising shifts and turns.
More famous metal bands like Iron Maiden and Metallica, which are not labelled progressive, also flirted with progressive elements on the albums "Seventh Son of the Seventh Son" and "...and Justice For All" in the late 80's, so maybe it came as no surprise the genre got a real breakthrough some years later.
A milestone of the genre came in 1992 when Dream Theater released their second album "Images & Words". The album was a surprise success that can be explained as a reaction to the new enormous popularity of grunge and various forms of alternative metal. Dream Theater sounded nothing like the alternative music at the time. The album is a pleasent and challenging roller coaster ride of instrumental passages, cathcy melodies and metal riffs - and with few exceptions the listener doesn't know what comes next.
Many bands later played in a similar style but the genre also has evolved into different types of progressive metal as the genre merged with other metal subgenres. Great examples are progressive death metal, progressive sludge and progressive metalcore. A lot of successful but very different sounding bands thus have appeared, including Tool, Messhuggah, Opeth, Mastadon and After the Burial, making progressive metal a very versatile subgenre today (especially when it comes to vocals styles).
Wanna read more? Check out this in-depth article from rocking.gr: https://www.rocking.gr/articles/The-Absolute-Guide-To-Progressive-Metal/17150
This thinking man's metal playlist will give you a decent introduction to the creative subgenre of progressive metal: