When does our technology become publicly know and not patentable?

We are considering applying for a patent. If I understand it correctly though, a patent application is defeated once the technology the patent is supposed to cover is publicly known. Is this correct? In any case, how "public" can we be about our product without defeating patentability? While we are certainly not planning to share our IP publicly, should we secure a patent before talking to -- for example -- investors/VCs?