Athlon - the official name
"Athlon" (aka Athlon classic, Athlon "A", etc.) - Slot-A Athlon with K7 and K75 core. Has 512KB of slower cache. You need the AMD750 or KX133 chipset motherboards.
"Thunderbird" Athlon (aka Athlon "B"😉 - Comes in Socket-A factor, but a very limited quantity comes in Slot-A that can be used with some AMD750 motherboards. Thunderbird Athlons come with 256KB of on-die L2 cache. They are generally faster than the "classic" Athlons. Use KT133, KM133 or KL133 motherboards for this CPU.
Newer "Thunderbird" Athlon (aka Athlon "C"😉 - Same as Athlon "B" but it's rated to use 266 MHz FSB, unlike Athlon "B", which uses 200 MHz FSB. Can be used with KT133, KM133, KL133, AMD760 motherboards at 200 MHz FSB, but it makes more sense to use it on KT133A, AMD760 or Ali MaGik motherboards at 266 MHz FSB with PC133 or PC2100 SDRAM.
Confused yet? 🙂