You have to start by understanding that for mobile phone companies, they are using an extremely specific and industry-focused definition of “data” that relates ONLY to the way mobile phone networks are implemented and billed.

If you are trying to understand it purely from any sort of more general, widespread definition of “data” which is what most people seem to be describing below, there are way too many steps and details between that and what the mobile phone company calls “data” for you to wrap your head around in a single question.

So I’m going to tell you what data means to a mobile phone company:

It means ANY internet traffic you use (upload or download) on your phone (or if you are sharing your phone as a hotspot, any used by the hotspot) AS LONG AS all of the following are true:

• That internet data is not for the purposes of sending and receiving phone calls to your carrier-assigned phone number across the carrier’s own telephony network (ie, it is a “regular” phone call, you have no control over how the carrier routes its voice calls but even if they do route it across the internet, typically you will not be charged data for this)
• It is not a SMS text message, and
• You are not on WiFi at the time (the WiFi goes through somebody else’s physical internet connection where the WiFi is connected to, not your phone’s).

There are exceptions and edge cases, but as a general rule, that’s what a mobile phone company will consider “data”. Anything you upload or download or stream on the internet almost always qualifies, unless you’re on a WiFi connection like at home or work, assuming you have that WiFi connection enabled. Youtube is data. Netflix is data. Emails are data. Phone calls can be data, if you’re using an “app” like WhatsApp or FaceTime or VOIP or any sort of video-calling feature.

It is measured in millions (mega) or billions (giga) of bytes. Text and static images, like wikipedia and many other webpages are, use negligible and almost irrelevant amounts of data. Apps, app and OS updates, streaming audio and especially downloading or even just playing games and video content (movies, TV shows, video calls) use very significant amounts of data and can quickly use up the quota in hours depending on the quality settings.