After basically reverse-engineering Intel's designs during the 1990s, AMD achieved technical and performance parity with Intel when they introduced the original Athlon in 1999. After that, the two companies traded the performance crown between improved versions of the Athlon and Intel's Pentium 4 until the introduction of AMD's Athlon 64, which completely dominated the Pentium 4.
The Athlon 64 was roughly 50% faster than the Pentium 4 at the same clockspeed, so the only way that Intel could compete with it was to lay a PR smokescreen and ramp up the Pentium 4's clockspeed to then-ridiculous levels, where they hit a brick wall, because the chips got way too hot. It was then that Intel realized that the Pentium 4's architecture was a dead end and that they needed to come up with something better. What they ended up doing was, essentially, to resurrect the Pentium 3 and continue to evolve its architecture (via the Pentium M) into what became Core 2. With the introduction of Core 2 in mid-2006, Intel took the performance crown away and AMD has never been able to take it back.
AMD tried countering a year later with Phenom, which was disappointing, as it was buggy at first, could not run at high clockspeeds and still fell short of Core 2's instructions per clock (IPC) performance. A year and a half later in Dec 2008, AMD launched Phenom II, which was a vast improvement over Phenom and generally matched Core 2, but by then Intel had already introduced its Nehalem architecture, and has gone on to release a new architecture roughly every 2 years, which is something AMD just can't match with its meager development budget.
AMD's Bulldozer, launched late last year, was just as disappointing as the original Phenom. It was actually slower
than Phenom II in anything but massively multi-threaded applications that took advantage of all 8 of its cores, as well as being power-hungry and hot. Piledriver, Bulldozer's replacement, is a marginal improvement that brings IPC performance back up to Phenom II's level, and, while still power-hungry, it doesn't seem to have Bulldozer's heat issues. IPC is still way behind Intel's, though, resulting in AMD's resorting to using higher clockspeeds in an effort to close the performance gap, much like Intel did with the Pentium 4.
a comparison of Piledriver and Intel's latest, Ivy Bridge, running at roughly the same clockspeed (the AMD turbos higher), showing the Intel architecture's clear advantage while using a lot less power. Note that while Piledriver has 8 physical cores, Ivy Bridge only has 4, but can run 4 more simultaneous threads (for a total of 8) through the use of hyper-threading.
On top of all that, due to, very poor management (IMO), AMD has been hemorrhaging money for several years, and I don't know how much longer they're going to stay afloat. It's all very sad, because it was pretty cool to see the little guy challenge and then smite the giant in the early 2000s, only to then become complacent and get his rear handed to him.