An event is a set of outcomes. "Rolling an even number" is an event: {2, 4, 6}. "Sum of two dice equals 7" is an event: {(1,6), (2,5), (3,4), (4,3), (5,2), (6,1)}.
Events can be empty (impossible), contain one outcome (simple event), or contain multiple outcomes (compound event).
To find the probability of an event, count how many outcomes belong to it. Next unit shows the formula.