As you may know, ROCKY started out as a script written by unknown, struggling actor and former porn star (with the wierdest damn face in Hollywood), Sylvester Stallone. The myth a lot of people perpetuate is that Stallone wanted to star in it, but the producers refused, so he didn't sell them the script and directed, produced, and starred in the film on his own. This fits perfectly with the ROCKY story arc: an unknown underdog that nobody believes in defies all odds and makes waves in the industry.
Unforunately, it's not entirely accurate: the producers did want the role of Rocky Balboa to be played by a big star, but Stallone appealed to them and because he managed to keep the budget around $1 million, the project was given the green light with John G. Avildsen (AKA not Sylvester Stallone) as director. The movie turned Stallone into a massive star, and he actually went on to direct all the sequels (except for ROCKY V which was directed by Avildsen, but I don't know that we can blame him for that).
If you don't know the plot of ROCKY, you are clearly anti-America, and therefore are the reason the terrorists won. Even if you've never seen it, trust me, you know the plot of ROCKY. It is exactly like every single underdog sports film released since then (of which I assume you have seen at least one), with the exception that Rocky loses his big fight at the end. And no, I refuse to apologize for spoilers in a review about a movie that came out almost 40 years ago. So basically, it's THE KARATE KID (also directed by Avildsen) but the main kid loses at the end. Seriously, if you watch THE KARATE KID right after watching ROCKY, you're just watching the same movie twice. Ralph Macchio is even from Newark, NJ, which is more commonly known as "Way Shittier Philadelphia."
ROCKY is awesome because Rocky loses at the end. Seriously, Apollo Creed is the undefeated heavyweight champion of the world, and Rocky is an unknown semi-pro boxer whose day job is collecting on debts for a loan shark and breaking thumbs (even though he's too nice to actually break anybody's thumbs). I gotta say there's pretty much no way Rocky would even survive that fight, even if Apollo Creed had completely stopped training before the fight (as is implied). It's Rocky's determination that allows him to go all 15 rounds, taking Creed's punches directly to the dome and just refusing to be knocked out. Poor guy's face should be absolutely fucke- ohh.
I love ROCKY because it made boxing popular again (remember boxing? You know, irrelevant sport, run by criminals, rampant corruption- no, not the World Cup, the other thing- fuck, not the Olympics, the one with only punching) for a while after its release. I also love it because I love Philadelphia (it's a great city and I love all their sports teams and shut the fuck up everyone). Even though Sly grew up all around the East coast (born in NYC, lived in DC, high school in Philly, college in Miami), he seems to genuinely love the city and you can really see it in ROCKY. Philadelphia has embraced ROCKY and it's become a huge part of the city's identity: the theme (Gonna Fly Now) is played before Eagles games, clips are played on the jumbotron at Phillies games, and Philadelphia institution Pat's King of Steaks (inventor of the Philly Cheesesteak) features a plaque on the sidewalk commemorating the film.
Photo by Pat Holland, outside Pat's King of Steaks, Philadelphia, PA
All in all, ROCKY is a solidly dudefest movie. It is the classic rags to riches underdog sports film, the one all sports movies that followed just fuckin' copied. It launched Stallone's career- without it we wouldn't have THE EXPENDABLES, and that's a world I refuse to live in.