Sum of physical evidence of the history of past life.
The fossil record is how we know literally what life looked like in the past. Fossils themselves, by contrast, represent the "physical evidence of past life". See also fossilization, i.e., the means by which this physical evidence comes to be preserved.
The means of preservation vary, but all involve brakes on the decomposition of all or, more commonly, part of organisms. Since hard parts such as bones and shells tend to resist decomposition best, it is those aspects of organisms that tend to fossilize most commonly.
In addition to actual parts of organisms, which can provide an indication not just of what organisms look like but also of their chemistry, far more common in the fossil record are casts of parts of organisms as well as material which has been either filled or replaced with rock crystal. Casts also can form from contact with living organisms, such as footprints, dinosaur nests, or worm trails left in mud, silt, or volcanic ash. These latter would serve as records of organism behavior. Even feces can fossilize, forming stones known as coprolites.
The likelihood that an individual organism will fossilize is immensely small. This is another way of saying that most organisms fully decompose upon death. The likelihood of fossilization is dependent on the size and hardness of the parts of organisms, the numbers of individuals that exist over time, the type of environments an organism lives in or finds itself in following death (eroding hills versus building sediments), to some degree their ecology, and not least of all luck.
Rapid burial of large numbers of organisms with fine muds and silt can lead to the most detailed of fossils whereas a failure to become separated from the atmosphere, or for aquatic organisms the water column, typically will lead to decomposition rather than fossilization. It is also crucial that a fossil be found prior to its being weathered into non-existence once it has been re-exposed to the atmosphere. The fossil record, as a consequence of all of these factors, is an imperfectly preserved record of life's past diversity and a highly biased one as well.