When Angelus was cursed with a soul and became Angel, Spike and Drusilla then went their separate ways from from Darla and Angel. It was when Season 2 of Buffy the Vampire Slayer began we were introduced to Drusilla and Spike as they constantly plotted the downfall of Buffy Summers but, of course, never succeeded.
When Angel returned to his evil self after a night of perfect happiness with Buffy, it started to drive apart the relationship of Spike and Drusilla due to jealousy on Spike's part. This would result in them fleeing from Sunnydale, although Drusilla had to be knocked unconscious by Spike while Buffy and Angel were doing battle.
It would not be until Season 5 of Buffy the Vampire Slayer and Season 2 of Angel that Drusilla would reappear in the form of flashbacks, plotting with Darla to return Angel to his former evil self, then returning to Sunnydale to start up with Spike again, even though he is now in love with Buffy.
Drusilla's madness and often very strange dialogue makes her an interesting character and shows just how evil she can be.