The clans and tribes are used to differentiate powers.
Demons. Two types: Scourge and Tempter.
Mages. Five paths: Enchanter, Necromancer, Theurgist, Shaman, Warlock
Vampires. Five Cclans: Daeva, Gangrel, Mekhet, Nosferatu, Ventrue
Werewolves. Five auspices: Rahu, Cahalith, Elodoth, Ithauer, Irraka
As for cross-compatability there is a three page appendix with conversion notes for the "normal" World of Darkness and on how to include OGL stuff.
Arashi, you can convert your Gangrel character to McWoD stats.
There is also a couple of pages with recommended viewing and reading materials. Blade, Neil Gaiman and Lovecraft are mentioned. The Underworld movies are not mentioned though.