You've seen tons of text symbols on Facebook, Myspace and YouTube. Special characters rose to popularity along with social networking. Most text signs, like ♥ aren't really used in books and references, but are easily recognisable graphemes.
Main reasons why symbols are rising to prominence are that they convey same meaning as words, but in a smaller space and they are well recognised among all internet users across the globe independent of their language, culture or ethnicity. Another reason is that they are often used as building blocks of text pictures to depict different emotions, concepts, images and other stuff (✿◠‿◠) You can type symbols right from your keyboard. I'm going to show you how. Also, if you want to check out all the symbols you have in each of font you got installed on your computer, check out Character Maps. ≧◔◡◔≦
Windows Alt codes
Shortcut technique that works on Desktops and some Laptops running MS Windows. You press Alt and, while holding it, type a code on Num Pad. It's very easy, but not as practical for long-term usage as Shift States. Also, you can type many frequently used symbols with this method, but not all like with Shift States.
Configure your keyboard layout in Windows so that you can type all additional symbols you want as easy as any other text. Takes about 5-10 minutes to set things up, but you'll be typing like a boss.
CharMap allows you to view and use all characters and symbols available in all fonts (some examples of fonts are "Arial", "Times New Roman", "Webdings") installed on your computer.
iOS Emoji keyboard
For Emojis (only) there is a special preinstalled keyboard.
MacOS Keyboard viewer
You can make frequently used technical non-fancy symbols like "√ ∑ π ∞ ∆ ™ © æ £ ¢" and åccénted letters on Mac using [Option] key. I've compiled a list of shortcuts in my article and explained how to open keyboard viewer. You can also use your Keyboard Viewer as an alternative to my list.
MacOS Character palette
Character Palette allows you to view and use all characters and symbols available in all fonts (some examples of fonts are "Arial", "Times New Roman", "Webdings") installed on your computer.
On non-handheld Linux (not Android mobile) there are 3 great ways to type Unicode symbols with keyboard. You can code all symbols with Unicode hex codes, type lots of symbols in the most easy way with Third and fourth level choosers, and easily compose useful signs with Compose key sequences. To find out how, read references that I made.
Character map allows you to view and use all characters and symbols available in all fonts (some examples of fonts are "Arial", "Times New Roman", "Webdings") installed on your computer. It can also help you lookup Unicode codes for entering symbols with keyboard.
As for emojis, modern versions of Android have emoji keyboard built into the standart keyboard.