This is a silly1 but rather useful for me. I always forgot the options sometimes.

Compress a files/folder:
tar -czvf <destination filename to be saved> <target files / folder to be compressed>
Example:
tar -czvf filename.tar.gz /var/www/

decompress files / folder:
tar -zxvf <compressed filename to be extracted> <destination to be compressed>
tar -zxvf filename.tar.gz /var/www/