A Preface
There are many sites out there and many books that can be read that will give you an absolute and in-depth look at the history of metal, but here is a brief look at some turning points and important events that have helped metal grow and mature as a genre. I suggest reading A Headbanging History of Heavy Metal by Ian Christe if any of this interests you.
Late 1960s-1970s
As with anything, we must first look at the roots of the genre. Bands such as Led Zepplin, Deep Purple, and The Who are often cited with having started the beginnings of what could be considered metal music. Although there is debate whether they were the first or whether they just influenced others to start, there is one band that is hands-down the definitive early metal band by any historian: Black Sabbath. The gritty industrial city of Birmingham, England, was undoubtedly the reason for the development of their revolutionary sound. The lyrics preached the same basic ideas of the flower generation before, but you could say they "motivated with a stick instead of a carrot". Their lyrics spoke of impending destruction and crumbling cities and the overall effects of war and the need for peace. This was something very new.
The Early 1980s
From Sabbath came many other bursts of metal groups throughout the world, the most notable being the movement started several years later known as the New Wave of British Heavy Metal, or NWOBHM. These young and energetic Brits utilized a vast increase in speed and technical skill. The range of lyrics was simply enormous, now ranging from simple love songs to in-depth historical pieces. Among the most well-know are: Iron Maiden, Diamond Head. Judas Priest, Motorhead, Raven, and a sea of others who wanted a piece of the new action. This generation had its run up until around the mid-80s. Although many of them continued far past then, (many still today) an interesting new sub-genre was emmerging that was newer, younger, and faster... much faster.
Mid 1980s
Speed Metal was the new king of metal. Finally, the US was catching on and was now a front-runner in the metal realm. The biggest place for this new breed of metal in the U.S. was the Bay Area. The "Big Four" came from there, and are still some of the most popular today: Anthrax, Megadeth, Metallica, and Slayer. They all had their own blend: Anthrax mixed thrash with elements of groove and oldschool hardcore punk, Megadeth showed the technical side mixed with straight-up thrash, Metallica kept it the most simple by borrowing from previous heavy metal generations, and Slayer took the cake for speed and aggression which would eventually spawn the entirely new genre of death metal. To trace all of the branches that these groups have influenced would take forever, you just have to know that almost every form of metal from here on was somehow connected to these four giants.
Late 80s-Early 90s
The next evoultion showed an interesting division. On one hand, bands such as Napalm Death, Morbid Angel, and Mayhem were pushing the limits in new genres of extreme metals in the new genres called: Grindcore, Death Metal, and Black Metal respectively. We saw this young generation trying with all their might to outdo their elders in every way. Whereas Anthrax and Slayer had left their NWOBHM forefathers behind, they were now having the same fate, only about 8 years shorter in between. Although older generations slowly evolved, this new breed was being almost forced, and it showed. Napalm Death has the world record for fastest band ever recorded to this day.
The 90s
The other branch was going to the technical side of things. Old bands were evolving to this progressive sound and new bands were emerging all over the place. Although Progressive music, or Prog, had been around for a number of years, metal was finally getting into it in the most extreme ways. Bands such as Dream Theater were pushing these new boundaries of technical prowess, and old trashers Celtic Frost were experimenting in such a direction no one could have imagined, utilizing orchestras and operatic vocalists. The floodgate was open and metal was now proving itself as one of the most open-ended genres the world had ever seen.
Today
Now we have such a vast ocean of subgenres, labels, and other divisions in metal that one could love one type of metal but totally despise another. Everything from Prog to Grindcore have expanded into dozens of sub-genres on their own and emerging genres are still popping up. It's an interesting future for metal, and a very uncertain one, too.