A parish is both an ecclesiastical and a governmental unit. There are more than 250 parishes in Cornwall.
Registers are simply books.
The recording of baptisms, marriages and burials in parish registers continues today, although of course only for those who are members of the Church of England. Once you determine in which parish your ancestor lived, you may be able to find the record of his or her baptism, death or burial in the parish registers. There is no guaranteed method. Some researchers are never able to determine this with certainty. Pay attention to all the details you know of your family.