Pascal's Triangle is a triangular array where each number is the sum of the two numbers directly above it. The edges are all 1s.
Row 0: 1. Row 1: 1 1. Row 2: 1 2 1. Row 3: 1 3 3 1. Row 4: 1 4 6 4 1. Each row n contains the values C(n,0), C(n,1), C(n,2), ..., C(n,n).
The triangle lets you read off combinations without computing factorials. Row 4, position 2 gives C(4,2) = 6.