has a detailed explanation of using connected storylines in Final Cut Pro X:
To learn more, also check out this tutorial from Streamingmedia.com.
In case you don’t know what storylines are with regards to FCP-X:
The Final Cut Pro X Timeline consists of two separate areas: the Primary Storyline, which is the darker area in the center, and everything else. The Primary Storyline is where you build your primary story, or all you’re A-roll, your main shots, your narrative–that sort of thing.
The areas above and below are for connecting clips for B-roll cutaways and shots that will support your primary story.