First a few definitions. Gravemarker is the generic term for a headstone, tombstone, or gravestone, normally carved from stone, placed over or next to the site of a burial in a cemetery. However, gravemarkers can be almost any substance including wood and bone. These markers are also referred to as stele (plural stelae). Wikipedia:Gravemarker. The problem is that there are a huge number of graves without markers. From the standpoint of a genealogist, relying on indexes, databases and transcriptions of gravemarkers is helpful but certainly not final. It is also not too uncommon to discover that a gravemarker has been placed arbitrarily and that the site of the actual burial is unknown. There are even gravemarkers for places where there are no graves or where the people buried are unidentified.
Overall, the various terms used by those who operate cemeteries vary from location to location. Here are links to two slightly different lists of terms used in cemeteries: Placer County Cemetery District No. 1 and Memorial Pines Cemetery. You can find hundreds of such lists online. For genealogical purposes, understanding these terms is crucial to discovering all of the information that may be available about a burial. If you don't ask the right questions, you won't get the answer you need.
As I have done more and more research in and about cemeteries, I have found more information that is evidently obtainable, but only when asking the right questions.
There are endless possibilities for what happens to people at the time of death. The body may never be recovered, the body may have been destroyed in the event causing death, the body may have been buried in an unmarked grave, and so forth. There may be a controversy as to who is buried in the grave, even with very famous people. But one thing is certain, there are a lot more records for genealogists to examine than are usually considered.
It is certain that a vast number of people in most civilized countries are buried in cemeteries or their equivalent. In the United States, most of these cemeteries are operated by some sort of sponsoring institution or entity, usually a local government, a church, or a private company. There are a significant number of cemeteries operated by or under the direction of the Federal Government also. In these cases, as you can probably guess, there is a lot of paper work associated with the burial. It is probably impossible to list all the variations in cemetery records because each of these entities have their own variations and forms. What is important to the researcher is to realize that these records must exist somewhere. Not all the records will or can exist in all locations, but there have to be some records somewhere.
For example, in Rhode Island there is a Rhode Island Historic Cemetery Commission that is a "a permanent advisory commission to study the location, condition, and inventory of historical cemeteries in Rhode Island and to make recommendations to the general assembly relative to historical cemeteries in Rhode Island." Nearly all the cemeteries, active, inactive, marked and unmarked have been inventoried and recorded. The The Rhode Island Historical Cemeteries Transcription Project has over 458,000 records for over 3400 cemeteries. Most of the states have similar projects.
Many beginning researchers ask for a death certificate as if this type of document was a universal key to identifying information about a death. However, in each state, death certificates are a relatively recent development, In Arizona, for example, death certificates were not required by law until 1906 and not universally implemented until sometime in the 1920s. This does not mean that there were not death records, it just means that the State government does not have the records. To find out more about the alternate places to look for death records, you may wish to start with this article in the FamilySearch.org Research Wiki: United States Death Records.
It is my further experience that there is a trail of records starting with the immediate records concerning the death such as hospital or other care facility records. There may be records concerning the transportation of the body, funeral arrangements and funeral programs, mortuary records, gravemarker purchase records, gravesite purchase records, permits from the city, county or state for the burial, documents recording the location and description of the burial, and records of other incidental expenses such as flowers or arrangement for military escort or police escort. Very, very few of most of these types of records are available online and most require some real detective-type work to discover. But they do exist.
The key, as I mentioned earlier in this post, is asking the right questions to the right people. Some further complications of the burial process vary considerably from country to country. One issue in the United States is the growing tendency to cremation. Although there may still be records, the disposition of the remains may be entirely informal.
The only thing I can say is keep asking and keep looking.