What Is Unicode? Behind the Scenes of Characters
A computer actually stores every letter, digit, and symbol you see on screen as a number; when you press the "A" key on your keyboard, the computer converts it to a specific numeric code, processes that code, and draws it back on screen as the letter "A". The set of rules that defines these numeric mappings is called a character encoding standard. In the early days of computing, ASCII was used for this; but ASCII could only define 128 characters (the English alphabet, digits, basic punctuation). That meant it didn't directly support letters like Turkish "ÄŸ" and "ÅŸ", the Greek alphabet, the Cyrillic alphabet, or pretty much any symbol.
Unicode was developed to solve exactly this problem. It covers almost every writing system in the world — Latin, Cyrillic, Arabic, Chinese, Japanese, Korean, ancient scripts — as well as math symbols, currency signs, arrows, and emoji, all within a single, shared numbering system. Every character is assigned a unique number called a "code point"; for example, the Latin letter "A" has code point U+0041, and the heart symbol ♥ has code point U+2665. Thanks to this standard, text written by a user in Turkey can display with the correct characters on a screen in Japan too, because both devices rely on the same numbering.
One practically important consequence of Unicode is that "real" characters and "image" characters behave differently. Most of the stars, hearts, arrows, and math symbols you see on this site aren't image files — they're Unicode code points, exactly like the letter "A". That's why they can be copied and pasted anywhere plain text is accepted — a chat box, a bio field, a spreadsheet cell — without any extra image upload or font installation. The only risk is that very old or very limited software may not yet recognize newly added code points (especially emoji that entered the standard in recent years); in that case the character shows up as an empty box or a question mark.
The Unicode standard is updated regularly, with new characters and new emoji added every year. The category lists on this site aim to bring together the most frequently searched and most widely supported characters based on the current standard, so you don't have to research one by one which symbol will "safely" work everywhere.