When do I have to use archive_command and when not

I have some doubts about PostgreSQL replication command. I am doing some research and I found severals configurations.

first one is:

archive_mode = on
archive_command = 'cd .'

second one is :

cp -i %p /var/lib/postgresql/9.1/archive/%f'

Then my questions are:

1. What is the different?
2. The first one option ( command ) is really strange to me. What do it do?