Birth, marriage and death records are normally abbreviated to BMD. You'll see this in many documents and legal writings. They are useful as they provide a summary of births, marriages and deaths throughout England and Wales from 1st July 1837.
When a birth, marriage or death is registered a certificate is filled out and the information is kept on file and added to the summary register, or Index, of all the events. As in books, the Indexes won't tell you everything, but will give you key information, such as names, dates and places. They also give you all the references you need to discover more details or to even order a copy of the certificate.
If you wish to obtain a copy for a birth, marriage or death certificate that was registered in your local area then you should apply by post or in person to the office which holds the record that you require. There may be a small fee for the administration and time though
The formal registration of births, marriages and deaths in England and Wales began in 1837 with the introduction of Civil Registration. Before 1837 individual parish was responsible for maintaining their own Parish register
While Civil Registration was meant to bring all birth, marriage and death information together in one place, it's important to know that registration was only made compulsory by law in 1875. So if you can't find your ancestors beyond that date you now know the reason.