Most computers these days come with a myriad of sensors to monitor the temperature of your computer. These sensors are generally located on the processor and the motherboard, and you might also have sensors on your video card. On top of that, all S.M.A.R.T-enabled hard drives have built-in temperature monitoring.
The temperature of your computer is a vital thing to keep track of - heat and computers don't mix very well. Unfortunately, Ubuntu doesn't setup your computer's sensors automatically; but you can follow these steps to enable the temperature sensors in your computer in Ubuntu, or any other version of Linux.