It's mostly data.
If you have iMessage activated, "texts" to iPhone users (who also have iMessage activated) will use data. They will not count against a text message "allowance". But they will count as data (albeit a small amount if you aren't sending pictures).
Messages to non-iPhone users will be sent as SMS and will not count as data.
You have the option to turn iMessage off if you are concerned about data usage.
Settings > Messages > iMessage > make the slider white instead of green.