In the surface, its main point is to define a strict notion of correctness in language usage, derived from mathematical logic, to disregard all metaphysical propositions as nonsensical and hail science as the only creditable source of knowledge. Wittgenstein assured to have solved all ancient philosophical problems by dissolving them: he pretended to ridicule and destroy philosophy by exposing it as a mere misuse of language and reduce it to a mere observer of science, thus claiming the final victory of the analytical movement (which his tutor Bertrand Russell commanded) over the continental tradition. This was the epistemological goal of the book; but it gets eventually attacked by he himself (remarkably, at 6.37X) and it's called off by the very end.
As Russell himself realized, Wittgenstein had a hidden agenda for this book: to deliver an ethical message (or rather, an ethical "example"). He wrote the Tractatus during WWI, down the trenches and dungeons; he wanted to talk about human life and our problems but, as he clearly acknowledges at 6.42, those kind of things are impossible to say "correctly" by the very means he had just defined; such is the "inexpressible" he talks about in 6.522; those are the "senseless" propositions of him that he denounces at 6.54 (and which comprise most of the proposition 6 entire set). We must climb up the Tractatus ladder to realize how useless logic is for actual life and so throw the ladder away: we must learn to speak correctly to realize that goodness is not something to talk about, but to live upon. We must learn to tell the time to speak (and do it correctly) from the time to shut up and act...
That is more or less my take on the Tractatus and what I see as the undying value of Wittgenstein.