The Angel's name is "Castiel".
As for the plot - Sam Winchester lost his gf, in the same way he lost his mom, which triggered him to want to find answers about what really happened to both his parents (his mom in particular) since she and the gf apparently werent the only cases to die the same way. He wanted to find out why was his dad was a douche, why was he a victim of "supernatural" misfortune etc. Dean (his brother) wouldn't let him go through the journey alone, so they traveled across the US in a Chevey Impala, killing every negative demon, entity etc they came across that harmed people, trying to help victims survive, while also looking for their dad for answers.
They got way more than they were hoping to bargin for, when Sam discovered what Lucifer and his minions tried to do his family, (trying to make Sam a vessel for Lucifer) and what they were currently working on. The story after that keeps evolving to include God, his angels and why they want to use Dean's body to go after Lucifer, should Sam fail to resist Lucifer taking over. And while all that is going on, more enemies (aside from demons) appear who also wanna destroy humans. Earth etc - so they have that to deal with too.
It's a pretty interesting and complex take to the biblically stories about how Lucifer (in the bible, not the show) tried to defy and attack "God", where Michael defeated him and the after life has always been in constant instability since.