Hey,
I've been playing since I was 12 (28 years ago) and have kids of my own.
There is a huge spectrum of Roleplaying Games. Wikipedia will help a bit to sort them out. Roughly they are; Live Action, Tabletop and Computer.
Computer RPGs are games like Skyrim.
Live Action RPGs involve some component of "acting" the character (a bit like theatre-sports). These range from simply describing, through dressing up and eventually to "offer" larps where (safe) weapons are used to resolve combat.
Tabletops involve saying what a character would do, and sometimes moving representations or figures on a map or board. Dice and published rules are used to resolve the chance of an action's success.
I'm going to assume here that your son is getting into tabletops?
There are literally thousands of tabletop games. Pathfinder is one of them, published by Paizo (who run this site). Most tabletop games have a "core" rules and a series of additional books that expand on these rules. Pathfinder makes all its rules available online through something called the SRD.
Tabletop games require a group to play and in this way are a social activity. Often these are groups of friends, but there are several clubs. Most stores that sell games will be able to tell you where the clubs are in your area. Pathfinder is more organised than most, and there is a global pathfinder playing club called "Pathfinder Society".
There was a lot of bad press from certain conservative groups around roleplaying games a few decades ago, but this was unearned and has largely moved on to video games as they grow in popularity.
Was there something in specific you wanted to know? Are you looking to play yourself? How old is your son? Obviously good advice is different for a 9 year old and an 18 year old!
How old is your son?
I would start with the rule book. don't expect to understand all of it on your first read through but you should get a good feel for it.
I think every game and every gaming group is a somewhat different. My gaming group consists of 30-40 year old male professionals. We sit around, eat junk food, tell stupid jokes, and roll some dice. It's very much a social game and everyone is trying to have fun as we explore dungeons, kill monsters and take their treasure.
Here are some 4th edition podcasts. It's a slightly different game with different mechanics but its a good starting point.
http://www.wizards.com/dnd/podcasts.aspx
I started playing when I was 8 or 9 and I turned out (mostly) all right.
Sounds like they already have a group.
8 is young to start roleplaying. I know people who have done it, but you may need to keep things a little simpler. If they are using adventures/modules I suggest you look at those first. The Core rulebooks themselves are usually pretty tame, but adventures (like books) range in appropriateness and there are some that have adult concepts in them.
If you are looking for a product aimed at a younger audience paizo recently released the Beginner Box
If it is not Pathfinder (or D&D) they are playing please shout out and we can try to tell you what the game is like. "Spirit of the Century" is pretty safe for kids that age. "Werewolf: the Apocalypse" probably isn't.
I would treat it like any other hobby your kid participates in. Be aware of what's going on, know his friends, etc. Participating is of course a great way to better understand it.
Another resource is to see if there are any local public groups that play games. There are a lot of game stores that host Pathfinder Society games, which are organized and structured. Sometimes stores run demonstrations of games too, to help teach them to new people. They can also make game recommendations.
Little Wizards is a game meant for kids ages 6-10. I've heard a lot of goods things about it. If you're not familiar with roleplaying, starting with something simpler that also includes advice on how to teach your kid could be useful.
Merlyn's is in downtown Spokane. There's also The Gamer's Haven in Spokane Valley.