Metal is a broad genre and not particularly easy to defineâ€”there are only a few things that are categorically true of all metal, and the rest are generalizations that apply to some subgenres of metal but not to others (e.g., many types of metal have fast tempos, but not doom metal!) These characteristics are generally true, but not always:
- Metal uses heavy, distorted guitars (often with downtuning); metal is heavier and more aggressive than normal rock.
- There is an emphasis on guitars: usually two guitarists, and songs often feature virtuosic guitar playing and guitar solos.
- Metal features use of nontraditional song structures instead of verse-chorus-verse. It also often features complex or odd rhythms, like syncopation or odd time signatures (this is more often true of death metal, whereas power metal often features a "galloping" rhythm, and doom metal's rhythm could be described as . . . plodding).
- Sometimes, but not always, there are blast beats in the drums and a double-kick pedal for the bass drum is used.
- Sometimes, but not always, there is a screamed or growled vocal style rather than singing. When metal bands have sung vocals, they usually tend to be what most people call "operatic," really a more traditional singing voice, often with vibrato. In contrast, the various types of hard rock often use either a sort of lower-register, sometimes gritty singing (Creed, Godsmack) or a higher-register sort of singing that is usually without vibrato (Green Day, Emery). Hard rock bands tend to not go for that rich, full singing voice that power metal bands favor.
Well, that is about the best I can do at defining metal; perhaps someone more knowledgeable will come along and clarify or correct what has already been said.
"Hard rock" is an even vaguer term than "heavy metal," and in my mind it could include all sorts of things, like alternative rock (Creed), grunge rock (Nirvana), screamo (Emery), or maybe even hardcore (Living Sacrifice). For the record, I would consider the Godsmack song you linked to be some kind of hard rock, not metal.