A numeral is a symbol that stands for a number. When you set an online password, it's safest to use both numerals and letters.
The actual symbol that represents a number is technically called a numeral, although many people use numeral and number interchangeably. You might have a watch with glow-in-the-dark numerals, or use the calculator on your smart phone to add up numerals. You can also use numeral as an adjective: "How many numeral cards are in the deck and how many face cards?" The Latin root is numerus, or "number."