Cellular data on the iPhone is data that is exchanged through LTE or 4G connectivity. This includes data communications, such as Web browsing, email, streaming music or video and push notifications, but does not include voice data.
Cellular data is covered under a separate plan with your wireless carrier, generally referred to as a "data plan". The data used when not connected to a Wi-Fi network is deducted from the plan's set amount. Instant messaging, music and movie streaming, as well as social media applications often use up a lot of cellular data. Turning off push notifications for messaging and email will help to conserve data when approaching the monthly limit.