Garmin has a free one called "opencaching" that I have tried.

... looks like there are 2 iPhone apps listed on geocaching.com

http://www.geocaching.com/live/default.aspx
GCBuddy
Geobucket

I haven't used either of them

OP here - thanks for all the responses! Can anyone give me a specific iPhone app to purchase? Thanks!

Very easy to do - we've started with our 4 yo. I have an iPhone and splurged for the $10 app, which was worth it as we don't have an actual GPS.

Try the iPhone app or a regular gps. It's fun and can easily be mixed into a day at the park for a picnic or long car trips.

Geocaching is like a treasure hunt for containers that usually hold a log book, trinkets for trade, and trackable items that want to move from place to place - you help move them around and log their whereabouts online. The containers are located it through a hand held gps or phone app which lead you to the latitude/longitude coordinates that the owner of the container (cache) posted online at geocaching.com. It is a fun and educational hobby. Sorry in a hurry and this is not well written, but check out geocaching.com, enter your address on the front page, and a list of caches will come up in your area - switch to map view - you will be amazed. Read all the rules on the site before embarking on your geocaching adventure.

Yes, we geocache with our daughter - started when she was about 7 in girl scouts and she's now 13. Great way to to spend quality time as a family. We also use a handheld Garmin. Another option to try is Letterboxing - the low tech treasure hunt
http://www.letterboxing.org/
We enjoy both.
Have Fun!

What is geocaching? What am I missing?

Yes, I started geocaching with my son when he was 12 - its really fun. I have a hand-held garmin, don't use the phone app, but you could try that.

Go to geocaching home page and type in your address or just zip, a list of caches will come up in that area. Try for one that is "regular" size and easy (one star for ease and one star for terrain). It's fun, we're hooked. It gets us out of the house for activity and we've been to nearby trails and parks that we didn't know even existed.

Anyone ever tried it with their kids? I want to and know that there is a geocaching.org site but it is so overwhelming! Anyone have a recommended iPhone app or other beginning tips for me? Thanks!