The GNU tar program does not know how to compress an existing file such as user-logs.tar (bzip2 does that). The tar program can use external compression programs gzip, bzip2, xz by opening a pipe to those programs, sending a tar archive via the pipe to the compression utility, which compresses the data which it reads from tar and writes the ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.