Task-based teaching is about creating opportunities for meaning-focused language use.
Task: An activity (or technique) where students are urged to accomplish something or solve some problem using their language. Preferably, this activity is open ended; there is no set way to accomplish their goal.
Task-based learning: Teaching/learning a language by using language to accomplish open-ended tasks. Learners are given a problem or objective to accomplish, but are left with some freedom in approaching this problem or objective.
(EFL glossary, Bogglesworld ESL)