Please note that most of this documentation has been adapted from Osmium documentation. Please see https://docs.osmcode.org/osmium/latest/osmium-file-formats.html for the original documentation.
OSM uses three types of files for its main data:
Data files: these are the most common files. They contain the OSM data from
a specific point in time. At most one version of every object (node, way or
relation) is contained in this file. Deleted objects are not in this file.
The usual suffix used is .osm.
History files: these files contain not only the current version of an
object, but their history too. So for any object (node, way or relation)
there can be zero or more versions in this file. Deleted objects can also be
in this file. The usual suffix used is .osm or .osh. Because sometimes
the same suffix is used as for normal data files (.osm) and because there
is no clear indicator in the header, it is not always clear what type of file
you have in front of you.
Change files: sometimes called diff files or replication diffs, these
files contain the changes between one state of the OSM database and another
state. Change files can contains several versions of an object and also
deleted objects. The usual suffix used is .osc.
All these files have in common that they contain OSM objects (nodes, ways and relations). History files and change files can contain several versions of the same object and also deleted objects; data files can't.
Where possible, Osmium commands can handle all file types. For some commands only some file types make sense.
Osmium supports all major OSM file formats plus some more. These are:
The classical XML format in the variants .osm (for data files), .osh
(for data files with history) and .osc (for change files).
The PBF binary format (usually with suffix .osm.pbf or just .pbf).
The OPL format (usually with suffix .osm.opl or just .opl).
The O5M/O5C format (usually with suffix .o5m or .o5c) (reading only).
The "debug" format (usually with suffix .osm.debug) (writing only).
In addition, files in all formats except PBF can be compressed using gzip
or bzip2 (add .gz or .bz2 suffixes, respectively - e.g.
data.osm.bz2).