I loved one masters team (super organized) and didn't so much love another. Swimming is a sport that you get out what you put in- period. From my experience, most masters team's have a dedicated coach or "group leader" who is usually a more experienced (or passionate) swimmer.
You can expect a semi-structured to extremely structured practice where you swim in lanes with people close to your ability level. The practices will likely very dependent on the groups goals and the coaches experience level.