SSH is a network protocol. It allows data to be exchanged using a secure channel between two networked devices.
Same protocol is used for:

ssh
sftp
scp

RPM packages required to rum ssh includes:

Openssh.rpm
Openssh-client.rpm
Openssh-server.rpm

The configuration files of ssh are:

/etc/ssh/ssh_config : for configuring client
/etc/ssh/sshd_config : for configuring server

When ssh is executed for the first time a file /root/.ssh/known_hosts is created.

This file contains public keys and IP addresses of remote hosts.
# ssh user1@remotehost.com Password-less communication

By the following setup a client should be able to access a server via ssh without typing a password every time.

Login to the clients box
# cd /root/.ssh (or which ever user)
# ssh-keygen -t rsa : Without entering anything keep pressing enter
The above will create two files id_rsa & id_rsa.pub
Copy the contents of id_rsa.pub to the server. Copy it into the home directory of the user as whom you will be login in. Copy it into /home/username/.ssh/ authorized_keys

Adding security to ssh

Users can be allowed or denied access to ssh by adding the following entries in /etc/ssh/sshd_config
allowusers abc : To allow abc
denyusers abc : To deny abc