Having spent many years in my working life in IT I know that data is the plural of datum and is a unit of information in its raw form, needing a program to transmit it computer to computer or to "read" it from a disc and process it into recognisable information.
But what is data on a smartphone in the sense that I get some "free data" and have to pay expensively if I use more (Greek tariffs).
How do I know when I am using "data" and how do I restrict how much I use.
In a mobile phone account context the data allowance is traffic - the amount of data (in the form of emails, images, HTML code etc. that you send or receive over the network you subscribe to.
How do you know when you're using data? The instant you start browsing on the internet you start consuming your data allowance. Each time you download an email you use a tiny bit more of your allowance.
When you use an app, more is used.
Each time you switch your phone on, apps start phoning home to see if there are any updates - more data is used.
Basically, your phone starts eating into your allowance when it's switched on, and doesn't stop as long as you are using it.
Save your allowance by turning unused apps off, or restricting their permissions. Don't turn GPS on unless you need it.