Data communication means sending and receiving digital data other than voice - text, images, video, app data - across a network. On a mobile phone, web browsing, app use, email, and video calls are all data communication. SMS is a short message addressed by phone number alone, but it normally travels by a different route (the mobile network's signalling channels, or the IMS platform used for voice) rather than as data traffic, so it does not eat into the "X GB per month" allowance on a plan; a per-message fee applies instead.
Mobile data communication uses packet switching. The data is split into small "packets" so that the line is never occupied continuously, which lets many users communicate at the same time. Traditional voice calling used circuit switching, holding a single path open until the call ended. That difference is why charging is split the way it is: calls are billed by time, data by volume. Plans set a monthly ceiling such as "X GB per month", and once you pass it your connection speed is throttled. After throttling, even text-heavy pages keep you waiting and video is effectively unwatchable.
The relationship between voice and data has shifted with each generation of mobile networks. In the 3G era, some combinations of technology could not carry data while a voice call was in progress. With 4G VoLTE, voice is handled as data as well, so browsing the web during a call became routine. 5G adds far greater capacity and lower latency, and is used for high-quality video calls and real-time remote operation.
To economise on data, lean on Wi-Fi, turn down video quality settings, restrict background app updates, and switch on the data saver feature. If your allowance is draining for no obvious reason, three culprits explain most cases: video autoplay, automatic cloud backup of photos, and automatic app updates.
One trap when choosing a contract: a data-only plan (a SIM with no voice service) cannot place calls to a phone number, and that includes the emergency numbers 110 and 119. Calling then depends on app-based IP telephony, but IP phone services on 050 numbers and app-based calling as a rule cannot dial emergency numbers such as 110 and 119 at all, so you need a separate way to call for help in an emergency. Our piece on LINE calls versus ordinary phone calls explains how data-based calling differs from a conventional phone line.