1. To know when to use tar command - if you have tar archive ( a file with .tar extansion) to untar it use
tar xvf file.tar
- if you wnat to create a tar archive. Use
tar cvf tarfile.tar file1 file2 ....
2. There is also a gzipped tar archive with .tar.gz and .tgz extansions
to untar them use
tar xvfz file.tar.gz
tar xvfz file.tgz
and similar to the previous to create the gzippped tar archive use
tar cvfz file.tar.gz file1 file2 ...
3. I can't read this question, please clarify.
4. For .gzip files use gunzip ( read man pages for different options) to decompress the archive
5. From the source means what it says, usually GPL sighned software is distributed with source available to the user, that means that the archive you downloaded contains actual code for the software, which you have to compile and install.