I am guessing you are talking about mobile phones. If you aren't, please correct me.
Signal : How close you are to a service provider hotspot, like EE (UK) and AT&T (US) . The closer you are, the better quality calls and data will be. If you have no signal, you cannot use data, calls or texts.
Fact : There is a mobile phone hotspot on Chomolungma (Mount Everest) !
Data : Basically internet, like wifi, except it runs through Mobile Signal, except much less powerful, and can be expensive. Do not use for video streaming, or large downloads!
Fact : The first data web browser was NOT on smartphone !