ADSL, or Asymmetric Digital Subscriber Line, provides internet connectivity through traditional copper wire telephone lines. It offers stable and reliable speeds, but the speeds can vary depending on the distance from the provider. On the other hand, 3G/4G mobile data is wireless and uses cell towers to transmit signals. It offers faster speeds than ADSL but can be less stable due to signal interference and network congestion. 3G/4G is ideal for mobile devices and offers flexibility in terms of location, while ADSL is more suited for fixed location devices.