How to people put those pretty letters and little pictures in their words?

[madison]

I always see all these things all over Twitter and Facebook of people typing with these really cool letters (like letters in bubbles, cursive, up side down, etc...) and with little pictures like crosses and cool little hearts (and I'm not talking about emojis!) Does anyone know how they do that and how I can do it? Is it like an app or something? Thanks.