Back in the original Pentium days, 1995, Intel was the only real company to make high end chips. AMd produced the k5 initially and it could not compete with the pentium. The next cpu AMd released was the k6, which had better performance than the original pentium but lagged in clock per clock performance compared to the Pentium MMX, which was also on the market at that time. Intel launches the pentium II and it once again obliterates the competition with its strong floating point processor. AMD responds with a re-spin of the k6 in the form of k6-2, that included 3dnow! instructions to help it perform better in the new arena of 3D gaming, as the k6 architecture had a very weak FP unit compared to the Pentium II. Intel also release the Celeron at this time and the Celeron A series, which was an overclocker's delight and was competitively priced to AMD's K6-2 series. At this time Intel still ruled the CPU arena, but AMD had made inroads as the value brand. Outside the gaming catagory, the K6-2 was a very strong processor, and did well for office duty tasks, which at the time was still the be-all end-all primary purpose of a PC. This was in 1998.
With the release of the pentium III intel was once again on a role, but soon AMD released the k7, named Athlon, and it was a very strong competitor to the PIII, and even reached the 1 ghz mark a day before the PIII. The two processors were very equal clock to clock, but the K7 ran much hotter and consumed much more power, requiring much stronger power supplies than previously used for the average home PC, as well as much more intricate cooling and faster cooling fans. On the values side of things, AMD's new value CPU, Duron, wiped the floor with Intel's Celeron. In general, AMD cpus were slightly cheaper than the competing Intel CPU. this was around the year 2000.
In late 2000 early 2001 Intel launched the P4 and initially it was not better than Pentium III. AMD launched the athlon XP processors a bit later that year and they were clearly superior to intel's Williamette p4. Intel then launched the northwood P4 and was able to increase clock speeds thanks to the die shrink to 130nm, and now Athlon XP and P4 traded blows on an even scale, with AMD's rating system being close to equivalent to Intel's P4 speed. Finally, intel launched the hyperthreading capable P4 in late 2002 and also broke the 3ghz barrier. It was at that point onward that Intel once again took the definitive performance crown, a feat they had not enjoyed since the end of the Pentium II days. AMD's Athlon XP simply could not scale up to meet the performance of Intel's P4'C' chips, and at that point the rating system of the Athlon XP did not match the Mhz speed of the P4. Essentially, a 2.6C P4 could best the Athlon xp 3200+ in everything except a few games. At this time Intel was up to 3.2 GHz. This was 2003.
In late 2003/early 2004 AMD released the K8 processor, Athlon64 and Athlon FX. It was quite impressive in performance and had efficient design with decreased power consumption as compared to the hot-running and power hungry athlon XP. The top end k8 cpu was able to best Intel's extreme edition P4 (a 3.2GHz P4 with L3 cache). From this point onward to the launch of Core 2 Duo, AMD held the performance crown, which is an impressive 2 years given the size of the company. Intel's flop with Prescott stalled out the p4 design, and when dual core became a hot ticket, Intel's P4 based dual core CPUs could only sell well with much lower prices compared to AMDs exorbitant costs for their Athlon X2 line. The situation did improve a bit with the launch of the 65nm CederMill P4 line, as now thermal were under controll and overclocking headroom improved, but by this time (late 2005/early 2006) Core 2 Duo was set to shake thing up, and the March 2006 IDF showed the power of Core 2 Duo, toppling the mighty Athlon X2 in one fell swoop. When the Core 2 line launched in July that year, AMD's once pricey CPUs dropped like a rock in the following months. The failure of the original Phenom (low clock speeds, high thermals, and poor overclocking headroom) enabled Intel to be undesputed performance leader for another year in 2007. Phenom II came along at the end of 2008/early 2009, and it offered some competitive peformance in the low and middle range of the CPU market but was not able to compete with Intel's new i7 series of CPUs. And that brings up to present, where AMD continues to languish in the middle of the market, waiting for bulldozer to fight Intel once more. The great news about AMD's current situation is that we all get some great cpus for seriously cheap prices, something that was also enjoyed in the K6-2 days, and at end of the Athlon XP days, and in 2007 with the demise of the Athlon64 X2. At each of those times AMD was at the losing end of the performance crown, but made up for it by selling some nice CPUs for relatively cheap prices that offered decent performance for the time. If AMD's bulldozer does indeed topple Intel, then expect CPU prices to rise once the old Phenom II stuff is phased out.