If you were anywhere near the bird-themed microblogging service last Thursday, you may have seen people tweeting about something called Sharknado. This campy mashup of marine-predator-meets-weather-disaster horror film was even announced as “trending on Twitter” Thursday evening.

This trend blip led a number of news organizations to take a closer look at the tweeting about the movie. The SocialGuide site, part of the Nielsen company, reported that Sharknado was the most discussed program on the air that night and generated more than 400,000 tweets, (even though the TV ratings urned out to be eh, not so much). But Sharknado made its mark on the trend lists and Twitter itself even blogged about the event.

So if you’re just a casual Twitter user, all of this trend business may seem a little confusing. How does a trend start? How do they count them? Is this just an advertising stunt?

Rest assured, trends are an official part of the Twitter service. In fact, when you’re logged into your account in the Web, you can see a little box of trends on the left side of the page and when you’re on the official mobile app, tap Discover then on Trends. There. You’ll see a list of keywords and hashtagged phrases. You’ll also see Promoted Trends, which are labeled as such and yes, those are advertising someone paid for. So, how did theses trends get there?

According to Twitter:

Trends are determined by an algorithm and are tailored for you based on who you follow and your location. This algorithm identifies topics that are immediately popular, rather than topics that have been popular for a while or on a daily basis, to help you discover the hottest emerging topics of discussion on Twitter that matter most to you. You can choose to see Trends that are not tailored for you by selecting a specific Trends location on Twitter.com.

So if you follow a lot of sports feeds, you’ll most likely see trending topics on that subject, like recent NBA trades if you have a team in your city. Hashtags are used in phrases, like #royalbaby to make the topics findable for people looking to search out all the tweets on the topic using that same hashtag. Trends also show world and local events.

If you want to see trends for other cities or parts of the world, click the Change link in the Twitter trends box to pick a different country or city and see what’s trending there. You can also type in “Worldwide” to get the global pulse.

When you click on a trend listed in the box, Twitter takes you to a page of search results for tweets using those keywords, phrase or hashtag. If you’re feeling like group activity, you can join in the trend by tweeting a new post of your own using the same keyboards or hashtag.

To avoid people gaming the system or trying to artificially inflate the importance of a topic, Twitter does have rules about trends, along with general rules for using the service.

As you can imagine, people love to see that topics are trending nationally or globally, and plenty of third-party sites that round up this info and present it in some sort of graphically pleasing form are out there. These include:

• What The Trend, a site owned by the social-media company HootSuite. The What the Trend page displays a list of Now Trending and All Trends Today, plus a handy list of top trends by country. There’s also a What the Trend Top Ten page and HootSuite’s Twitter Trends of the Week.

• The Hashtags.org site has page called Trending on Twitter that looks at the hot hashtags of the moment.

• Trendsmap, a geographical real-time representation of what the popular topics around certain parts of the world. You can sign in to Trendsmap with your Twitter account as long as you’re doing it for personal, non-profit and research use only. Otherwise, you can sign up for Trendsmap Plus at $19 a month to get ad-free data you can filter by hashtag, keywords or users, faster updates and Vine videos.

If you’re a marketing or advertising person, you can also find paid services for analyzing Twitter traffic for your business.

And if you missed the original airing of Sharknado, Syfy is rerunning it this Thursday at 7:00 p.m. Eastern time. You can find clips from the film around the Web (spoilers, darling!) if you don’t want to sit through two hours of giant toothy wind-propelled fish chomping their way through L.A. But then again, maybe you do.