Science is just us accumulating more and more knowledge and getting a more and more accurate picture of the world, right? Not according to Kuhn, in The Structure of Scientific Revolutions, first published in 1962. Yes, there's progress, in terms of better and better answers to a given question, more and more data collected in pursuit of a particular undertaking, but different areas within science periodically reformulate their questions and redefine what their undertaking consists in. These "paradigms" within science are something like new traditions: a particularly interesting scientific work comes along, such as Newton's Principia with its laws of motion and all that, and bam, the questions that research is supposed to answer, its methodological assumptions, and its picture of what constitutes rigor are established, and so scientists in that area (which shouldn't be confused with one of the named areas of science like "physics"; a paradigm can cover a much narrower area with only a couple dozen professional practitioners, such that folks outside of that area might not even understand that a profound change has occurred) en masse refocus their efforts and begin filling in the blanks left by this new picture of the world. Folks that hang on to the old paradigm are redefined as obsolete, or maybe not scientists any more (maybe they're philosophers!), or maybe practitioners of some other science.
This sets up Kuhn's distinction between the progress of "normal science," i.e. that operating within a paradigm, and scientific revolutions, which mark the change from one paradigm to another. What begs for philosophical clarification here is the notion of a paradigm, and Kuhn despite his efforts is light on details, as he was not actually a philosopher by training, but originally a physicist and then a historian of science. He does make it clear that a paradigm is not a matter of a set of explicit rules that a scientist working in this area is supposed to follow. Following Wittgenstein's Philosophical Investigations, Kuhn uses the word "paradigm" to indicate the exemplar in question (the work of Newton in the example above), but also uses the term to indicate work that's kind of like that, where "kind of like" signifies more of a family resemblance than something that can be formulated into an explicit set of rules. Kuhn tries to unpack the various definitions of "paradigm" more explicitly in his 1970 postscript to the second addition, and readers might even want to start there and then read the essay.
He also raises in that postscript the issue of what others have tried to do with his idea of a paradigm: to carry it outside of science. Kuhn sees this as ironic, because it was really his innovation to bring such a notion from places like art where there are well-recognized changes in fashion based on admiration of famous/revolutionary artists. Even so, it's still a leap requiring philosophical clarification, which Kuhn does not give, in order to apply the notion to philosophies themselves, or more generally, "world views." Yes, there are some questions that our current vocabulary in some area of philosophy might not contain words for (thus we get folks like Heidegger making up words or diving back in etymological history to find them) or evidence (e.g. of miracles) that we would not under ordinary circumstances accept. Certainly, also, the idea of a weltanschauung or Deleuze's use of "plane of immanence" is related to Kuhn's talk of paradigms. Still, Kuhn himself has a very narrow range of phenomena in mind, namely the practices and assumptions involved in doing science, and his story is not readily transferable to other areas without a lot of work.
Kuhn stressed that even though he could not describe the progression of paradigms as progressions toward a more adequate representation of truth, he did not consider the movement between paradigms as arbitrary. Not everyone might be convinced and jump to the new scientific paradigm, and with every paradigm advance there is a loss of some advantages even while there's a gain in other areas, but the progression could still be considered "progress." He compares his position to Darwin's theory of natural selection as opposed to an older view of goal-oriented evolution. To go from one paradigm to another is to adapt to a state of crisis within the old paradigm, where the phenomena that the paradigm left unexplained led practitioners to come up with ad hoc and varying extensions of the paradigm to explain it. So there is a progression: Newton laid out a powerful new way to make precise predictions, even as his theory reintroduced "occult properties" that had been routed out by the previous paradigm. (Newton had no explanation for gravity: as with Aristotle, it was merely a tendency that things exhibited and not something with a really explained cause.)
So we can understand the logic of the progression, and so call it progress. What we can't do is say that it had to progress in just that way, or predict how it will progress next, or draw a neat line to say that now truth is being more nearly revealed. Kuhn distinguished between experience (like the experience of experimenters) and the stimulus for experience. We can theorize about what might be causing experience, but ultimately our only access to it is through those theories, those representations, which employ the vocabulary and other elements of a particular scientific paradigm: there is no theory-free observation. This means that, without too much dramatic exaggeration, we can say that scientists operating under a different paradigm are experiencing a different world: the stimuli of course have not changed, but the conceptual categories and such that actually make up our world, the world of our experience, have changed. This should sound very familiar to anyone who's up on our discussions of Kant; this is just a sort of evolutionary variant of this, where we don't have a set of conceptual categories hard-wired into us, but where at least some of these are determined by the paradigm, i.e. by culture. Whether or not Kuhn successfully avoids charges of relativism or whether we should care about that is up to you to decide!