Wow, you really are innocent :-)"Yield to temptation, as it may not pass your way again." Spoilers refer to material about or from an as yet unaired episode. They can be concrete material from legitimate sources like casting sides or scripts, or unsubstantiated material from setside sources, stunt doubles, people in post-production, and the like. Tabula Rasa members (in general) love spoilers. We adore them. We are a listful of Spoiler Whores. Nonetheless, our spoiler-phobic friends would greatly appreciate a warning in the thread title if you're going to talk about spoilage in a post. To get the goods, see Wendy's SpoilerZone on this site.
So named because they "spoil" an episode, these include any details regarding the plot of a future episode. Spoilers are usually denoted by a "spoiler alert!" In fanfic, authors typically list any episode spoiler warnings prior to the text of the story.