Crossfit is good for nearly anyone of any weight. The overall goal is "Elite Fitness" which is basicly to be the best you can be. The workouts range from ones for testing strength to ones focused on endurance. Every day is a new workout. This way you're not just doing 12 reps of X exercise for so many sets. Going to the gyn doing that can be so monotinous.
The workouts are posted at
Welcome to CrossFit: Forging Elite Fitness and you can go to the Start Here>BrandXScaled workouts to get a scaled down version if you're unable to do the one posted. You don't have to do the one posted for that day, you can go to the FAQ section and just pick a "Hero" or a "Lady" workout (they're just names of workouts, not anything to do with sexes.)
You can look through the Workout of the Day (WOD) section here and see some of the past prescribed workout to get an idea of how they're done. If you have any question about the execution, youtube has a ton of Crossfit videos, but be sure to get form down before you try adding the weight or doing quick reps for time.
The guy said he's crossfit certified? that means he's been to seminars and knows how to teach and do the workouts effectively. I'd love to get a cert. for it as well, but unfortunately they've got no seminars in my area coming up, plus the ones in surrounding states for Level 1's are all sold out.
Good luck! Crossfit is awesome!! Once you start, you'll think your old gym routines are boring. Even if you don't do it for a long duration, Crossfit is definintely a change to help mix up your current routine.