Cancellations are the marks used to obliterate part of the design of a stamp to indicate that it has been used and is no longer valid for postage. Like many other aspects of the hobby cancellations form just another side road that some collectors decide to travel.
Various types of cancellations are described below
"Hand" Cancellation is one that is applied by a rubber stamp. It is usually the name of the town processing the item and the date of processing. Early versions sometimes consisted of a number representing the location within a simple design. Some collectors actively seek out items posted from specific locations
"Manuscript" Cancellation is one that is applied by pen and ink. It can be as simple as a couple of lines scrawled accross the stamp but is often found as a signature of a person at the post office handling that item. It is sometimes accompanied by a date. There exists stamps where the person has become famous and such stamps can be extremely valuable. This practice of cancellation has all but ceased
"Cancelled To Order" This type is a carefully applied hand stamp only obscuring one corner of the stamps design and is most commonly found on stamps that have not been used in the postal system but have had a cancellation applied for the collector market
"Commemorative" Cancellation is one that celebrates a significant event or milestone and it may not even relate to the subject matter of the stamp. The variety is vast as individual towns will apply a commemorative cancellation significant to that locality. Thes could celebrate famous birthdays, anniversaries of discoveries, bridge openings or anything at all.
"First Day of Issue" Cancellation is one that is applied on the very first day that the stamp goes on sale to the public. These can be very plain or even elaborate and can vary from one post office to another
"Base" Cancellation is one that is applied to mail emanating from a remote base. Australian Antarctic research stations are one example - the Australian Antarctic Territory stamps are valid for postage within Australian borders but are of significantly more interest if cancelled at one of the research facilities.
"Machine" Cancellation is one that is applied by the automated sorting machines at central mail handling facilities. Usually simply a date and obliteration bars.