Ok here goes:
In the United States there are a ton of universities. These universities have sports programs. The universities have decided over time to form leagues. The main leagues (main primarily because of the popularity of the sports program ie. fans and because of the size and history of the university) are the Pac-12, Big-10, Big-12, SEC, and ACC. The names of these conferences are historical (why does the Big-10 have 14 teams? Well they added 4 teams and didn't change the name, etc).
These 5 conferences are colloquially referred to as the "Power-5". The reason for this is money, prestige, number of fans, history of the individual teams in the conferences, etc. . These 5 conferences are joined by 5 other conferences (MAC, AAC, some others) and they are colloquially referred to as the "Group of 5" because historically they are not great sports programs and because some other things relating to the relative academic and financial strength of the universities.
Each year, these 10 conferences play 1-4 games against each other (A Big-10 team might play a team in the MAC for example) and then the rest of their games are played against other teams in their conference. In the end, there is typically a conference championship game (the two best teams* in the league play each other to decide who the champion is).
Now that's the easy part, the hard part is the rest but here goes:
Every year, there is a poll of the associated press and the university coaches. One is called the AP poll and the other is called the Coaches poll. They are
completely irrelevant and are historical, but people pay attention to them because until around the 6th week of the season they are the only quasi-official rankings that matter.
Around week 6 a new poll comes out. This is the college football playoff poll. This poll is the only poll that matters. It's a committee of some number of individuals (I don't recall) who get together, take into account all possible factors, and release a ranking of the top 25 teams in the country.
At the end of the season, the top 4 teams play in the college football playoff. The rest of the teams are invited to "bowl games" which are basically just for fun and for fans, but they do generate revenue for the sports programs and everybody gets to have a fun time. The reason there are a lot is basically because they are for fun and for the fans, and everybody leaves happy. This is also a good chance for programs that might not typically play each other to get a chance to play each other. Although it happened in the playoff, the first time Georgia and Oklahoma played each other just happened.
At the end of the day, from the perspective of an alien, the only 3 games that matter are the college football semi-finals and the championship game. But from the fans perspective, bowl games against highly-ranked teams (USC-Ohio State or Penn State - Washington for example) serve as a benchmark for the performance of the conferences and bragging rights.
Hopefully that helps - and if anybody wants to add on please do. I'm trying to be a bit unbiased here as well. If you believe what I've said is biased please let me know and I'll fix it. Maybe other people could add links to the conferences and teams and what not and turn this into a sticky.
