PS is a little extra note you add after you have already finished a letter. You have signed your name — then one more thing comes to mind, so you write PS and the new sentence. It is short for postscript. The idea is “after the writing.” People still do this on paper, and they do it in emails and phone messages too: a last thought that did not fit the main body. If a second extra thought arrives, some people write PPS, a half-joking second postscript. The tone is ordinary, even warm — a PS is often the human bit, a reminder, an “I forgot.” You may see it with dots (P.S.) or without (PS); both are fine. Though actually, those two letters do other jobs in English. On a games box, PS is PlayStation. This entry is the letter one. In Taiwan, people often just write PS in a Chinese message; they have borrowed the letters as they are.