Emojis (from Japanese 絵文字) are pictographs 🍋 (pictorial symbols) that can be used in text, just like other characters. The way they look 🕶 might change from one browser 🦊 to the other. Emojis are vector-based, so you can scale ↕ them as big as you want. They stay the same for every font 🈂.
For HTML-encoded characters like μ, ⇑ and «, check the HTML character map