they're all different models of LEDs with different characteristics.
To summarise;
XR-E - the "oldest" emitter, still respectable output with the benefit of excellent throw due to the very small emissive surface and integral reflector. the "least" efficient LED still in use
XP-E - "Modern" equivalent of the XR-E, cheaper due a considerably smaller package, can throw just as well but requires a different reflector to do so. now capable of better efficiency than the XR-E
XP-G - upgrade of the XP-E and XR-E, with a larger die (the light-emitting bit) is capable of greater efficiency and substantially improved total light output. moderately less throw due to the larger emitter.
XM-L - biggest single-die LED that cree make with a 2mm x 2mm emitter-surface. capable of double the current of an XP-G, but at four times the Die-size, has substantially less throw. Currently the most efficient LED in use.
SST-50 - this is an LED manufactured not by Cree but by Luminus Devices. capable of more current than the XM-L but less efficient by about 15%
SST-90 - Another Luminus LED, even bigger than the SST-50, capable of running at more than 5A iirc. still less efficient but brighter overall than the XM-L