Psy in "Gangnam Style." Youtube Youtube turned 10 years old last week.

In the past decade, the website has become a huge part of our culture and an important platform for brands to engage with users.

It's where music videos live. It's where movie trailers premiere. It's where a whole new class of multimillionaire "Youtube stars" have cropped up.

To chart Youtube's growing influence, Business Insider talked with Youtube expert and virality consultant Brendan Gahan.

Gahan is the founder of social media marketer Epic Signal, and his current client list includes PepsiCo, Anheuser-Busch InBev, Microsoft, and Uniqlo.

Here are the 10 most important videos in Youtube history, with Gahan's comments:

1. "Me at the zoo" (2005)

This 18-second video of Youtube cofounder Jawed Karim standing in front of elephants at the San Diego Zoo started it all. "It's amazing how unassuming the first video on Youtube is," Gahan says. "In a lot of ways it's representative of Youtube — it doesn't need to be this fancy production; it can be approachable. The first Youtube video is something anyone could create on their own."

2. "Lazy Sunday" (2005)

Saturday Night Live stopped being cool sometime in the 1990s. Then "Lazy Sunday" happened, in which Andy Samberg and Chris Parnell rapped about buying candy and seeing "Chronicles of Narnia." The video went bonkers on Youtube before it was taken down. "Lazy Sunday was one of the first times where Youtube went everywhere," Gahan says. "It was like a viral moment for Youtube as well. Instantly that player was everywhere you went."

3. "Pokemon theme" (2005)

The Youtube star universe started with Smosh, the name of comedy duo Ian Hecox and Anthony Padilla. In total, their parody videos about video games and pop culture have amassed 6 billion views and 32 million subscribers. It all started with their singing the Pokemon theme song. "It was right when all the coverage around Youtube was coming in, right before the Google acquisition," Gahan says. "It really propelled Smosh to stardom."

4. "Sunglass catch" (2007)

This Ray-Ban produced video was one of the first viral ads. Blurring the line between real and fake, the video shows two guys throwing sunglasses at one another from atop a bridge or into a moving car, with the wayfarers magically landing on the other's face. "It was a lot of stupid tricks," Gahan says, "but they did really well, and the videos were an a-ha moment for brands." Marketers realized Youtube could reach big audiences.

5. "Justin Singing So Sick by Ne-yo "(2007)

Before there were any Beliebers, Justin Bieber was a Canadian kid whose mom uploaded a video of him singing R&B. That video was spotted by music promoter Scooter Braun. The rest is record industry history. "It's probably the earliest example of someone coming from Youtube into the mainstream and just dominating," Gahan says. "For someone to be plucked from relative obscurity and essentially dominate the music industry, that's pretty incredible."

6. "Yes We Can — Barack Obama Music Video" (2008)

Youtube showed its political potential in 2008, when Will.I.am gathered a bunch of celebrities to sing Barack Obama toward the White House. "The Youtube community rallied around Obama," Gahan says, "and the power the platform resonated within the political community, where before that it was largely ignored."

7. "Japan Earthquake: Helicopter aerial view video of giant tsunami waves" (2011)

In the same way that Twitter became the place where news breaks, Youtube has become the place where news videos often first surface. That trend started with the 2011 Japanese tsunami. "It seemed like an event where people were getting more stuff online and watching the Youtube videos before it was surfacing up to TV," Gahan says.

8. "PSY - GANGNAM STYLE" (2012)

At over 2 billion views, "Gangnam Style" is the most-watched Youtube video of all time.

"Gangnam Style was the Macarena of the post-YouTube era," Gahan says. "It was goofy and catchy, and something you could share to get a few laughs. Soon so many people were sharing it, it was weird if you hadn't and you almost felt pressure to join in by sharing. It was social proof at its finest — you had to share that video at that time because everyone was."

9. "Bill Gates ALS Ice Bucket Challenge" (2014)

The "ice bucket challenge" took over the late summer of 2014, where videos tagged #icebucketchallenge uploaded from over 150 countries received over 1 billion views. It raised $98.2 million for the ALS Association in just one month.

"Being in advertising, you heard it brought up in every single meeting," Gahan says. "Something of that scale is near impossible to replicate for a brand. It was for a good cause, and the whole thing of calling people out and putting a time limit on it was very smart, since it creates urgency."

10. "GloZell's Interview with President Obama" (2015)

In 2015, Youtube stars got access previously reserved to elite journalists: a sitdown with the President of the US. Makeup artist Bethany Mota, comedian GloZell, and self-proclaimed geek vlogger Hank Green each did interviews with the president. It's an acknowledgment of the medium's maturity: "[Obama] recognizes how insanely powerful these people are and the audiences they've built," Gahan says.