When Buffy is killed by the Master, she's brought back to life by Xander in a natural way. When Kendra dies, that calls Faith to the Slayer line. What this essentially means is that while Buffy is a Slayer, Faith is the Slayer, the one who possesses the lineage. This is why when Buffy dies again at the end of season 5, no new Slayer was called. Where it gets tricky is when Willow uses magic to resurrect Buffy, which causes a split in the Slayer line. They've meddled with forces far beyond their control, ones that shouldn't be meddled with, which leads to a big chance for evil to capitalise on it. This means that the First Evil was able to come back in season 7, and attempt to wipe out the line of the chosen one, thanks to the irregularity caused by bringing Buffy back from the dead.