There is currently no required certification for teaching yoga nor is there an organization monitoring the quality and authenticity of yoga teachers. Anyone who wants to can invent a new brand of yoga and market it as a new lineage. Teachers' training programs vary and can range from a weekend to two years.
Several schools of yoga have strict certification programs, and there is a process underway to set up some type of certification for yoga teachers. Many teachers have a certification and have gone through a comprehensive accredited program Others are not certified but have taught yoga for many years.
As with any certification program, a yoga certification denotes that the individual has studied and obtained a certification at a particular school of yoga. On the other hand, many good yoga teachers are not certified, but have extensive or inclusive training.
You certainly want to make sure that the instructor you choose is knowledgeable and understands how to teach yoga. But, the most important.