Las Vegas doesn't have a high or low season by the standard definition, but you'll find it least crowded between November and January. Hotels are at their fullest July through October. Specific events—New Year's Eve, Super Bowl weekend, spring break, March Madness, major conventions—draw big crowds, so plan accordingly.
It's well known that summer highs often exceed 100°F, but with low humidity and ever-present air-conditioning, you can stay comfortable as long as you limit your time outside to short intervals. Even in the hottest months (late June through September) you can bear the heat, provided you stay hydrated and don't try to walk too far. And with more than 300 days of sunshine a year, the chances of a rain-out (or snow-out in winter) are slim.
On the other hand, it does get chilly between late fall and early spring. There actually is a winter here, with nighttime temperatures of 40°F, and sometimes, though rarely, below; so bring a sweater or windbreaker for your evening strolls beneath the neon-bathed skies.