Garden formal is fancy wear that is not inappropriate for outdoor terrain. Women should wear dresses or a nice shirt/skirt combo but pay attention to length. You won't want to wear anything too long...
Well, sometimes it depends on the person hosting the event, so it would be safer to ask, but typically it means that Formal attire is going to be the norm, but is not required... so you could dress...
Informal attire could be a skimpy bikini for an attractive girl, sunbathing at the beach. But she wouldn't wear that to her graduation ceremony, or to her office job.