A decision tree is a flowchart-like structure in which each internal node represents a test on a feature (e.g. whether a coin flip comes up heads or tails) , each leaf node represents a class label (decision taken after computing all features) and branches represent conjunctions of features that lead to those class.
A decision tree has the substantial advantage of forcing the evaluation of all conceivable outcomes of a decision and tracing each path to a conclusion. It generates a complete analysis of the effects along each branch and flags decision nodes that require additional investigation.
A decision tree makes its decision by running a series of tests.
The ID3 (by Quinlan) algorithm is the fundamental method utilized in decision trees. The ID3 algorithm constructs decision trees in a top-down, greedy manner.