ANGEL:

An Irish gentleman born in the early 1700's, Angel, or Liam as he was known then, was more interested in drinking than doing an honest day's work. It was on a drunken binge that he met Darla, who turned him into a vampire.
It was in 1898, after he killed the favourite daughter of a Romany tribe, that they restored his soul, forcing him to live with the torment of the the acts he had committed as a vampire.
In the early 1990's Angel moved to California and decided he would help the Slayer in the fight against evil. He and Buffy fell deeply in love and on the night they consumated their relationship, the Romany curse was lifted and he lost his soul.

SCOTT HOPE:

Buffy breifly dates Scott but he remains an 'in-school' boyfriend. When it appears they are finally connecting, he gives her a friendship ring- a claddagh like the one Angel gave her and Buffy panics. He asks her to the Homecoming dance but senses something is wrong and doesnt actually take her.

PARKER ABRAMS:

Parker is Buffy's first crush at college, he seems to be everything she could want in a guy. However, Parker isn't as he seems, after sleeping with Buffy he takes off.

RILEY FINN:

Riley is Buffy's first proper boyfriend at college, although there is no immediate signs of romance between them.
Riley is more than he seems, a high ranking officer within the Initiative and when Buffy finds out, they end up working together. The relationship breaks down when Riley is caught being fed from by a female vampire.
He believes that Buffy has never really loved him although he is so deeply in love with her. He takes off and when he finally returns to Sunnydale, he is married.

SPIKE:

A vampire born sometime in the late 1800's possibly early 1900's. When he originally came to Sunnydale it was to kill the Slayer. Only after being captured by the Initiative and having a behaviour modifying chip implanted in his head does Spike become a 'good guy'.
Spike first realises his feelings for Buffy when he wakes from a dream that they are together. Their relationship starts out rocky but eventually they get together, although it seems its pure lust rather than a deep and meaningful romance.
Buffy eventually reveals that she is using him. Desperate to win her back, he goes to her home to try and make her feel like she used to with him resulting in attempted rape on his part. Horrified at himself, he leaves Sunnydale in order to have his soul restored, hoping that will convince her of how much he loves her.