It is necessary for a wireless mouse to have a transmitter inside it and a receiver installed on the sytem to which it is (to be) connected. Line of Sight communication is not needed in this case, as required in infrared communication. The receiver and the sender works on the radio frequency usually the microwave frequency i-e 2.4 GHz. At such a high frequency, the chances of interference is almost negligible. These frequencies also provide a range of almost 150 ft which is sufficient for a hand-held mouse. What happens is, that the RF signals sent from the transmitter are received at the receiver and the decoded and given to the processor to execute the task.The wireless technology in mouse are mostly accomplished on the optical mouse and not on the old mechanical ball mouse of early 1970's.
Do you have any idea about 802.11b or 802.11g ? These are also related to the wireless technology and the protocols that I will discuss later.
Lets talk about the advantages of the wireless mouse, it has over the others. First of all, it provides you a user friendly environment - no wires - no headache. Secondly they are not powered by your system supply, they have their inbuilt rechargeable or disposable batteries. Thirdly the wireless mouse do not require Line of Sight communication and thus provide you a feasible environment Lastly they are inexpensive and and you can buy one for less than US$ 25.