What OS do Nokia phones run (Symbian, Maemo, or something else?)? What language and toolkit do I use? Is it locked-down, or can I load apps on to my phone (or m

What OS do Nokia phones run (Symbian, Maemo, or something else?)? What language and toolkit do I use? Is it locked-down, or can I load apps on to my phone (or my friend's phone) that I write? I basically have a good idea what the answers are to those questions for iPhone and Android. No idea for Nokia.