How to become certified in an area of nursing

I heard that you can become certified in an area of nursing such as ob/gyyn, a lot of my instructers has this type of certification. Just wondering how dose that work and can a nonob/gyn nurse work there or do they have to be certifed first before doing ob/gyn work?
As an RN, you can work as an RN anywhere where an RN license is required. Your instructors more than likely have their master's in their specialty fields... which is the MSN and you choose a specialty area such as Midwife, Peds, Adult, Psych. There are also Clinical Nurse Specialists and this also is a Masters
To become "certified" in a specialty, you need to meet the requirements of the nursing organization offering the certification. Usually, the requirement include:1. An academic degree (Some require BSN's or higher, but I don't think that all do)2. Work experience in that specialty (usually a couple of years)3. Continuing education credits related to that specialty4. Pass an exam (similar to NCLEX) that focuses on the specialtySpecialty certification is a way to demonstrate advanced knowledge in a particular type of nursing.