http://www.google.com/hangouts/
I've been seeing more and more websites like this. Since I don't do much web programming, I was wondering what this is. Is it a specific library, or just a new design?
Where would I start if I wanted to update my websites with this style?
I don't see particular use of framework here, only their own CSS (just maia.css), so I guess is just a new design wit tons of div with different background colours.
If you don't like bootstrap and you want something "cool" with a vertical design you could give a try do Adobe Muse, which unfortunately comes only with the Adobe Creative Cloud subscription. 
I'm not sure what aspect you are referring to with a "long" site.
I've seen some that rely on the css "flexbox" that allows boxes to resize and shift and reflow based on the browser window sizes. I've seen some that include multiple iframe elements to pull in other web pages on the client side, or composition of everything on the server side, or a bunch of scripting to allow for infinite scrolling by loading in new html fragments.
Personally I dislike the style. I use plugins that disable scripts and cross-site loading by default, which has a positive side effect of not loading ads and other annoyances. Most "long" sites rely on javascript that invariably pull their scripts from sites outside their control, a few of those like googleapi I mark as trusted, others not so much.
I'm not sure what aspect you are referring to with a "long" site.
I don't really know what to call it. I've seen more and more of these where the background images span the entire screen, there is some parallax background scrolling, and different sections. But there are similarities, such as a menu bar that appears after scrolling down a ways, and click-scrolling items that I figured it was some fancy javascript library or something I hadn't heard of.
Spotify's website is another example. I'm not a fan of the style either, I was just curious.
That's called parallax scrolling. To obtain it without JavaScript you need to use CSS3 transitions and transforms/transform-style (which could be not supported by opera mini and internet explorer prior version 11).
Some tutorials:
http://css.dzone.com/articles/cool-parallax-scrolling-effect
http://blog.keithclark.co.uk/pure-css-parallax-websites/
https://ihatetomatoes.net/how-to-create-a-parallax-scrolling-website
Imo parallax scrolling could be used for good and little homepage presentation if it is not too long, or in a credits/about page.. Unfortunately some oompa-loompa... ehm designers without a simple knowledge of what memory and bandwidth mean, tend to use huge and poor compressed background images .-.
The things that seem to be "en vogue" right now(?) are Parallax backgrounds and responsive design. ie. One layout that adapts to every possible screen width in a more or less nice way.
If you mean infinite scroll, personally I detest it.
Since we all hate them so much, that means they will become the new "Gold Standard" and everyone will have them very soon.
Wonderful...
My favourite way to make my browser crashIf you mean infinite scroll, personally I detest it.
I really dislike parallax scrolling on websites. I find it very distracting and makes it hard to focus on the actual content. Please don't ever do it.
