the command is as follows:

tar -zxf file name(with extension .tgz)

for more information u can see the man page for tar

gunzip filename(with extension .tgz)
then
tar -xf filename(with extension.tar)

for zipping particular file 
gzip -v xyz.txt  or bzip2 -v xyz.txt
for extracting zip file (.gz) or (.bz2)
gunzip -v xyz.txt.gz
or bunzip2 -v xyz.txt.gz