"Что такое жизнь?"
Translation:What is life?
See tips'n'notes: кто is masculine, что is neuter. It's kinda tricky though, because you say "кто такая Анна?", for example (who is Anne), but you say "Кто пришёл?" (who came?), even you do know that it is a woman. With что you have always neuter, though: Что это? (what's this?), что случилось? (what happened?) etc.
the first part of the sentence "Что такое?" means what is it. you can put any word after this to ask a philosophical question like the one here: what is (it) life.
IF you write "Что такая жизнь?" you say something like: what is such life. "такая" is now a demonstrative pronoun describing one spesific life, and not life in general.
Well, without any context it could be used only like I wrote above. In a daily speech, one usually says: что такое жизнь? Or что есть жизнь? (In a general meaning of the word "life"). But the second one is less common.
In Russian, the verb "to be" is ommited almost always. The interrogative pronoun is just enough. Look: Что такое химия? Химия - это наука (What is chemistry? Chemistry is a science). Где ты (есть)? (Where are you?). Откуда ты (есть)? (Where are you from?) Когда (будет) обед? (When is the dinner? / When will be the dinner). Он - вор (He is a theft).
In English we definitely should use "to be" forms in such sentences and it causes some difficulties for Russians who learn English))
It could mean "that life" but only as a part of a compound sentence: он думал, что жизнь не закончится никогда = he thought (that) life would never end.