The end of the series in season seven is open to interpretations, which was probably the best way to end it. It gave ammunition to both Spuffy
and Bangel shippers to use in the battle of "Who does Buffy ultimately end up with?"
Basically, Angel guest stars on the last two episodes of Buffy to give her the amulet that will consume the hero who uses it to save the world. This is where Buffy gives Angel the famous
link where she explains that she's not sure if she's ready for long-term planning or commitment as far as relationships go.
As for Spike, well, (spoiler) he DOES technically die in the season finale of Buffy. He wears the amulet, and, as previously mentioned, is therefore consumed by its energy in order to save the world. However, in an interesting twist, Lindsey McDonald (from Angel the Series), mails the amulet to Angel's office at Wolfram and Hart in Los Angeles, where Angel opens it and unintentionally resurrects Spike as a vampire ghost. For half of season five of Angel, the Fang Gang scramble to figure out a way to make Spike corporeal again. In the meantime, the banter between Spike and Angel is priceless.
I highly recommend finishing Buffy, or at the very least watching Angel, which (in my own humble opinion) is better than Buffy after season three.
In the comics, Buffy and Angel belong to different companies (Dark Horse and IDW respectively). Because of this, neither Spike nor Angel can (officially) cross-over into the comic Buffy universe, because different companies have rights to the characters. However, that doesn't mean that Buffy can't have a scintillating dream about Spike and Angel in "Long Way Home" (have I mentioned how much I want to blow up that image into a poster and put it on my wall? I LOVE IT!)