spontaneous writing

Hi,

how does a computer recognise spontaneously, when a person writes on a pen tablet/PC tablet.
Does it have to do with multi-threading concept?? ie....if a person writes a word "HAPPY", the moment he writes the character "H" and when he moves to write "A" on the pen/pc tablet,the display would have shown the character "H" or sometimes the entire word...

can anyone tell me the concept behind it??