What do you want to know about outdoor Free Flight? Have you looked at the Small Flying Arts website? Free Flight Mailing List on Yahoo?
OK, some big differences to indoor.
The airplanes are much stronger, though weight is still very important. The lightest classes are 7-10 grams, many more than 20. P-30, a very popular event, has a 40 gm minimum.
Planes use rubber to climb to maximum height, and competitors launch to catch a thermal for maximum time.
As a result, the flight pattern and trim is very different from indoor due to desire to climb to maximum height and then extend glide.
To keep them from flying away, they often use dethermalizers to end flight after a prescribed time.
There are many classes, but like indoor break down into a couple of big categories. Scale (scored on flight time and scale points) vs sport (scored on duration only). Inside sport it breaks down to AMA events (tend to be simpler planes, though not always) vs FAI (think formula 1 racing) events.
If you like scale, the places to start are no-cal, for FAC dime-scale. Simple high wing planes to start.
If you are more into duration, consider Embryo or P-30. Both very buildable and flyable events.
Investment is higher than indoor, but not crazy until you get to the very competitive FAI level where individuals planes can run over $1000 each. A good P-30 can be built for $30 from scratch, kits from $25 to $75 or so.
A good book to start with is Don Ross's Rubber Powered Model Airplanes
The organizing body in the US is the National Free Flight Society http://freeflight.org/
Anything more specific?