Is it taken by both pilots and flight attendants? How frequently? What is the main purpose of it? Is it mandatory?
Recurrent training is required annually by the FAA for both pilots and cabin crews, by 14 CFR 121.427. I am only familiar with the content of recurrent training for flight attendants so this answer is limited to them only. In a previous answer I wrote:
Annually we would be required to attend jet recurrent training where we reviewed NTSB accident reports with an emphasis on cabin crew responses to emergencies, review of first aid and CPR techniques, re-testing of our knowledge of the location and how to use all the safety equipment on board, including life preservers, life rafts, and emergency slide procedures. We were also briefed on any changes to FAA mandated procedures and tested on everything.
The purpose of the training is to refresh our knowledge of required safety procedures that might have gotten rusty and to stay up to date on possible changes to FAA required procedures. Emergency procedures are almost never needed and yet it's vital to be able to recall them when needed. One day a year was devoted to recurrent training and you had to pass all the tests to stay current. For this reason, we took it very seriously.
Recurrent training must be conducted regularly, under most rules (EASA, FAA etc) 2 simulator sessions twice a year. It is mandatory, basically to prove to authorities that the pilots are still up to handling emergencies according to the airline handbooks.
Because airliners are so safe nowadays, nothing happens. My apologies for oversimplifying this, but pilots fly from A to B and back to A again year after year, and nothing happens. So then they find themselves in a Level D flight simulator, and everything happens: engine fail, dual hydraulic fail, smoke in the cockpit, alarms going off, "woop woop Pull Up!"
It is a very useful requirement for keeping everybody sharp and on top of things. The exercise is not an automatic pass - upon a fail a pilot can get a re-assessment. Fail this again and continued employment can get hairy.