Let's say you're asking about a science experiment. The answer to "Ποιο ήταν το αποτέλεσμα;" would be something like "the water evaporated". The answer to "Τι ήταν το αποτέλεσμα" would be something like "The result was obtained after 10 hours of combining various chemicals in a laboratory under standard scientific conditions..." Apart from a few exceptions, like time, ποιος is generally used for easily-answerable questions and outcomes. τι is used for asking more fundamental questions about the nature of things. The difference is quite subtle and took me a long time to learn, and I still get it wrong a fair bit, but a useful rule of thumb is that if you could respond to a question with a response of 10 words or less, use ποιος. If you could respond to the question with endless waffle, use τι.