Cisco's Internetwork Operation System is the operating system that runs on a majority of Cisco hardware. In the case of CCNA R&S it'll be running on routers and switches.

IOS operates in 3 main command modes to accomplish config tasks.

The first mode, also known as guest mode. This is the mode you'll be in if you fresh boot a device or remote in with telnet / SSH. The User EXEC Mode prompt is a single Router>

The second mode, also known as enable mode. To enter Priveleged EXEC Mode issue the enable command from User EXEC. This mode hosts a majority of show commands in IOS and is where you perform restarts and save configurations to NVRAM. The prompt for this mode is an octothorpe: Router#

The third mode. To enter Global Configuration Mode issue the configure terminal command from Privileged EXEC Mode. This is the mode you'll be in for doing most configuration tasks. It's from this mode you can branch off into it's submodes to configure specific interfaces and settings. Config Mode prompt is config enclosed in parens followed by an octo: Router(config)#

Issuing exit will take you to the previous mode. Example:

Router(config)# exit
Router#

CTRL-Z will immediately throw you back into User EXEC mode from any config mode.

? is basic help. Entering ? at any point during a command or even at a blank prompt will give you a list of available commands. Use it often.

Router(config)#interface fastEthernet ?
  <0-9>  FastEthernet interface number

Tab completion is available and short commands are allowed:

Router(config)# interface fa<TAB>
Router(config)# interface fastEthernet

Router(config)# int fa0/1
Router(config-if)#

While in Global Configuration Mode or deeper, prepend do to run Privileged EXEC commands:

Router(config)# show run         # Doesn't work!
Router(config)# do show run      # Will work!

** ADD OUTPUT MODIFIERS