How to know something previously unknown, by using something known.
Inductive inference, when we make a prediction that generalizes observed data, but doesn't logically follow from it. See Statistical inference, Learning theory
Deductive inference, when we find a statement that logically follows from observed data.
Abductive inference, starts with an observation then seeks to find the simplest and most likely explanation. See Causal inference, Causality
Complexity, Phase Transitions, and Inference by Cristopher Moore (part 1)
Other classifications and types