Like so much in this hobby there is a long and a short answer. At the risk of giving more than you need or are intested in:-

Mandatory registration of births and deaths (marriages are slightly different but basically the same) began in in March 1878.

Voluntary registration had existed since March 1849 when it became legal to perform baptism and marriage in other than the established church.

The Registrar general maintains a record of all registered births and compiles an index on a yearly basis. These indices (to about 1960) are available on LDS films. (Unfortunately, many of the marriage entries give only the husband's name.)

In addition, the actual Registrar's books for the years 1884-1911 are availabel on LDS films. This is the material the Registrar will use to issue a certificate.

Also registered births and deaths post 1848 are also available on LDS films.