Quote Originally Posted by Aluette View Post
"Записывать" ("записать" - perfective) means that you write (or record) some particular piece of information - записывать телефон (to write down a phone number), записывать сообщение (to write down a message), записывать диск (to record a disc).
"Писать" ("написать" - perfective) means the process, means that the size of the text you are writing cannot be valuated exactly - писать письмо, диктант, роман (to write a letter, a dictation, a novel). Though in colloquial speech you can say "Пиши телефон!", but only in imperative.
Some more nuances.

If I asked someone to write his/her phone number for me, I would say:
Напиши номер своего телефона. (not "запиши") - His/her phone number is unknown to me.

If I asked someone to write down my phone number, I would say:
Запиши номер моего телефона. ("напиши" is still possible, but less natural) - It implies I ask him/her to write down something what is predefined and known to me, and it usually assumes that I dictate (or show) my phone number, and he/she writes from my words or what I show.