Kind of like a standard Shakespearean tragedy ending, except there isn't a secondary character that turns up to wrap up the story after all the main characters die off, and all the preceding story is usually highly fluffy in order to heighten the impact.
*Hides MoonShadow behind a pastel sparkly curtain* Nope, nothing to see here. Ignore the man hiding in the statue speaking into the microphone...
CLAMPEndings are nice, happy things. Yes, yes. Nothing but peace, love and happy endings. Yes.
Please move along. These aren't the droids you're looking for.
These aren't the bishounen you're looking for? Hmm. How many kinds of ending does Clamp really have? There's the classic/tragic TB ending, the fluff CCD ending, and the they-made-a-holy-mess-out-of-this-manga ending (RayEarth? 2?). So not going into English major poisoning babble to explain the whys and wherefores.