Permutations vs. combinations

An easy way to remember the difference between permutations and combinations, two terms which are often used interchangeably but incorrectly:

Don’t memorize the formulas, understand why they work. Combinations sound simpler than permutations, and they are. You have fewer combinations than permutations.

Discrete Mathematics was one of my most challenging classes at UVA, where seemingly simple concepts (e.g. the basics of counting) turned out to be not-so-straightforward.