Ni is an introverted perceiving function, and is associated with imaginative premonitions of near-absolute certainty for no discernable reason. "I have a feeling that X will happen, I just know." Dominant Ni types are often stereotyped as insane conspiracy theorists or psychics. Ti is an introverted judging function, and acts as a sort of subjective, impersonal categorisation of all things. It's often associated with "finding the right word" for something, e.g. what obscure genre of music a song might fit under.
To me, Ni is more about shifting your perspective. A basic example of this (lazily drawn from what someone said in the Ender's Game topic) would be to walk "down" the street, and then to think that from other viewpoints you're walking up, left, right, and so forth. You've framed your perspective inside another one that transforms how things appear to you. It's when you pair this with Je that you're most likely to think in terms of the future. Je orients itself toward goals, and so when Je engages Ni, or vice versa, Ni will think in terms of how the future frames the present, which then allows you to act accordingly.
Ti doesn't have much to do with that. What Ti does is attune itself to physical and abstract structure. A Ti-user will call on a mental sorting system, such as logic or a sense of aesthetic harmony, and use that as a tool to structure their thoughts and notice structure in the environment. What doesn't fit with that sense of order--logical errors, for example--Ti will critique and attempt to re-categorize.
There are only two places where the processes really cross. The first is that they can both lead a person to develop a system for viewing the world. In the case of Ni, this system will base itself on perspective and transcend the mundane (think of Gnosticism, which advocates liberation from the world). Ti, on the other hand, will create a system based on logic and order (think of physics). That's the first point of similarity. The second point of similarity is that these processes both lead a person to develop inner imagery. The difference is in the type of imagery. Ti will visualize the structure and flow of a system in a rational and orderly manner. For Ni, the images will cover a wider range of territory, from the mundane to the dreamlike, and they won't make a lot of logical sense. In many cases, they'll answer a question you didn't even ask, and in others they'll simply transform how you view something.
Actually, there's one more point they have in common, now that I think of it. Both types of thinking can lead to skepticism. An Ni-user will wonder if they aren't being mislead by appearances (which apparently makes them prone to conspiracy theories), and a Ti user will question whether something is true according to good logic (they usually don't consider how subjective logic, as an introverted process, actually is). Those are two very different brands of skepticism, but they can look a lot alike, even to the person using them.