tar format doesn’t compress data. “Compression” here refers to making files smaller (less disk usage) without losing informartion by using stuff like e.g. repeated patterns or the fact that a text file doesn’t use a lot of non-alphabetic characters, etc.. Especially Linux/Unix users will often encounter
.tar.gz file endings (or similar). The
gz ending comes from gzip compression, basically a tool that can compress single files, so you’d turn something like
tar format on the other hand only has the purpose of combining multiple files (e.g. a directory or a directory structure) into a single file; that’s why these two formats compose pretty well. People turn a directory
foo into a
foo.tar archive and then compress this single file into
tar library you linked says it only handles the
tar part of this practice, so no compression. (Also, of course, you don’t always need to do the compression step. Especially with small files nowadays compression might be pretty redundant,
tar alone can help out for things like – for example – sending a directory over email, etc.)
For comparison, other archive formats like e.g.
zip do both compression and saving a directory structure in a single file.