What's the difference between ANOVA and t-test?

A t-test tests the significance of the difference between two group means. For example, comparing test scores between 2 classes.

ANOVA (Analysis of Variance) tests the difference among three or more groups by seeing if at least one group is significantly different. For example, comparing test scores across multiple classes. In summary: use a t-test for two groups and ANOVA when three or more groups are concerned.