Extracting files from a tar file

[Raymond Yee]

This should be a simple question (although I can't figure it out.)  I have
a tar archive file sitting on my disk and I would like to extract the files.
How do I do it?  I've tried a mix of the x and f key options, but I
constantly get a tape read error.

I'd greatly appreciate any help I can get.  TIA.

Raymond Yee
(ryee at ocf.berkeley.edu)

PS.  How would I collect a group of files into a tar archive?