While the punk "sound" predates the Ramones by many years, it is commonly held amongst music circles that punk rock as a genre wasn't properly founded until the New York band's self-titled debut album in 1976. From the get-go, they set the standards for punk culture--the often in-your-face attitude of their songs and lyrics deterred many radio stations and labels from approaching them.
However, as their July 4, 1977 concert in London demonstrated, that didn't keep them from being the bedrock of a burgeoning punk movement. The London concert, attended by future members of the Sex Pistols, Sousxie and the Banshees and the Clash (sometimes called "the only band that matters"), birthed the British punk scene and set the tempo for all punk acts that followed.
From the start, punk rock was defined by the simplicity of its music--the Ramones' limited repertoire and ability helped coin the term "three-chord wonders," emphasizing their music's constrained range and rhythm, and often short vocal phrasing. The priority, rather, was on the aggression of the sound; the Sex Pistols, in particular, became famous for discarding skill entirely, often playing out of tune, inebriated, high or all three at once. Yet the vicious and nihilistic, often yelled-out lyrics of Johnny Rotten were thought to represent far better a generation that felt itself burned by the disappointments of the 1960s.
It is, however, the deliberate constraint of punk rock that is its musical character, not always the nihilism of its lyrics. The Clash, in particular, found it unnecessary to sacrifice musical ability for political activism, combining technical sophistication and influence from other genres to culturally significant effect.
As with punk rock, the origins of heavy metal can be traced back to the blues and rock genres dominant in the 1960s. From there, however, a sharp split occurs--Led Zeppelin, often considered the grandfathers of heavy metal, were quite adamantly technical musicians, even if they shared the Ramones' love of the lead guitar, rhythm guitar, bass, drums and singer setup. So, too, was blues band and prototypical heavy metalists Black Sabbath, infusing the genre with the now common themes of darkness and occult fantasy.
The birth of what is now considered "thrash" metal occurred around roughly the same time and place as British Punk was starting off, with both genres accentuating the aggression of their culture. Heavy metal, however, chose to do so through their technical play--at times sacrificing even vocals to do so.
Heavy metal's technical characteristics stresses its electrical instruments--it is best known, in particular, for highly complex guitar melodies, often exploiting techniques only available via amplification and distortion of magnetic pickups. Later on, speed became more and more of an emphasis--combined with complex riffs and harmonics, heavy metal became a genre that stressed technical virtuosity.
Though quantitatively small, modern bands such as Darkthrone, Motorhead and Celtic Frost have attempted to combine qualities of both heavy metal and punk rock. The synthesis of the two otherwise distinct genres tend to emphasize around the quality (or lack thereof) of the studio recording, aiming for a dirtier sound than is usually embraced in heavy metal. In the music itself, elements of punk rock rhythm and progression can be found in non-symphonic chord sequences.