The presently available remote controls can be divided into three basic types: lower end remote controls, mid level remote controls, and higher end remote controls. Lower end remote controls do not have the facility of universal remote control. That is, if you purchase a lower end remote control, it can be used only for a particular device manufactured by the same manufacturer. But most of the mid level remote control and all the higher end remote controls have this facility to be converted into universal remote controls. You need the code of another manufacturer's remote control to enable its functionality in your remote control.
Take the above example of Sony TV, Sanyo VCR, Samsung DVD player, and Philips music system. If the remote control of Sony is a higher end product, you can convert this remote control into a universal remote control to operate all the devices you have. For that, you need the universal remote codes of the remote controls of other devices provided by their respective manufacturers. So if you know the codes of Sanyo, Samsung, and Philips remote controls, you can feed that codes to Sony remote control, and make it a universal remote control.