There once was a vampire. He killed a lot of people. He only tortured Slayers. Back then, his name was William the Bloody. Cecily turned him. After Angelus turned a girl, he fell in love. Nobody knows what her name was as a mortal, but her vampire name was Drusilla (Dru). He fell in love with Dru, but Angelus loved her too, even though he already had Darla as his soul mate So William the Bloody and Angelus fought over her for centuries.
Several years past and his name was shortened to Spike, because he would kill and torture Slayers with rail spikes from railroad tracks.
After he and Dru left their creators, they brought chaos to Sunnydale. That's when they found out that Buffy was the Slayer; that Sunnydale had its own Slayer. They tried several times to kill and/or torture her, but never succeeded.
Several years later, after Angel and Cordelia left town, as well did Dru, he and Buffy fell in love.
Later, he joined Angel and his crew in L.A, knowing that Buffy thought he died in the fire that happened in Sunnydale. And that's his story.