I'm interested, sell me on it.
Basically it works like this. Paizo created PFS as a way for people to create and play their characters at any PFS game without having to worry about having a set group or table. You have to register your character on the site and you're good to go. You start off at level 1 and gain 1xp for every scenario and 3xp for every module you complete. Every 3xp you level up (there is an option to change your character to level up every 6 instead if you want). Every time you complete an adventure, you receive a 'boon' which gives you access to minor abilities and such.
As a player, you can only receive credit for participating in a scenario once. So if you have one character that did X scenario, you can't get credit/xp for having a different character run through it as well (There are a few exceptions to this, but they're mostly lvl 1-2 stuff). There are a TON of scenarios out there.. Over 200 at least I think. What I personally like about it is the fact that I can and have taken characters from local games and taken them online or to cons.
Now, they did put a few restrictions on characters for balance reasons. If you want to take a look at what's allowed and what's not, take a look at this site
Archive of Nethys. Anything that has a little symbol by its name is PFS legal. This book
PFS Guild Guide goes into a bit more detail about the rules and such.
