Heavy Metal is a particular guitar sound/tuning. That's all. The first metal song was without a doubt,
You Really Got Me, by the Kinks. They however didn't follow down the path they accidentally created, but many bands heard that song and ran with the sound. The biggest names to do so were Deep purple, Led Zeppelin, and Black Sabbath.
As music and the technology to make it evolved, many bands incorporated metal into their sound or the sound of some of their material. Not may would think of the normally Acid Rock band, Pink Floyd as a metal band, but they certainly used elements of metal in some of their pieces (Have a Cigar, Young Lust).
In the 1980s metal really took off and split into various segments with some diverging greatly from the original sound. There was the Speed Metal of Van Halen, or Joe Satriani, the Thrash Metal of Anthrax, the Hardcore of Slayer, and Metallica, the Glam Metal of Motley Crue, or Gunz n' Roses, the Death Metal of Cannible Corpse, the Industrial Metal of Skinny Puppy and Ministry.
The 90s saw a mash-up of many musical styles plus far more digital technology and electronic music, and the coming into mainstream of Rap and Hip Hop. About midway through there was a recording revolution made possible by the cheap technology, and Alternative was born, and most of it was metalish. Metal was very mainstream. Somewhere along the lines there were revolts against revolts. Some bands came along with that old Deep Purple sound, and we got Retro Metal/Stoner Rock, with such faces as Queens of the Stone Age, Fu Manchu, and High on Fire.
So metal is everywhere these days and it's quite hard to separate. If a band uses electric guitars, they will probably incorporate at least some elements of metal in their works.