Goth rock is a sub-genre of post punk music which originated in the UK in the late 70s (specifically 1979).
The first goth song is said to be "
Bela Lugosi's Dead" by Bauhaus because even though the song was supposed to be written tongue-in-cheek, the band performed it with naive seriousness and the audience understood it with seriousness.
Goth rock characteristics include a 4 / 4 post punk beat, a drum machine, baritone vocals (common in men, like
Peter Murphy of Bauhaus and
Andrew Eldritch of
The Sisters of Mercy),
flanging guitar used for atmosphere and a prominent bassline. Some goth bands use characteristics of tribal music in their work, too (see: Southern Death Cult and Stairs - Uncertain Journey by Christian Death).
Contrary to belief, goth has not "evolved" and throughout the decades, the sound has relatively stayed the same. However, other dark or spooky genres and bands have cropped up throughout those decades which commonly get called "goth". This is false as goth only specifically refers to bands within that genre, not any band that uses a "dark" image or "spooky" lyrical themes.
Bands like Marilyn Manson and EBM bands like God Module are commonly mistaken for "Goth" when they're part of different genres.
The sound for all three of these groups are completely different, therefore could not be considered to be part of the same genre.