A conjecture is a proposition that is unproven but appears correct and has not been unproven. For example Goldbach's conjecture, which states that every number greater than 2 can be written as the sum of two primes.There is a great deal of numerical evidence to imply that it is true, but it is still unproven.
A hypothesis is a proposed explanation for an observation, and is stronger than a conjecture. It is directly testable, and often serves as the starting point of an experiment, which seeks to show that it is either true or false.
A thesis is an unproven statement put forward as a premise in an argument. For example Church's thesis in logic proposes that every computable function is computable by a Turing machine.
A theory is an explanation of reality that has been thoroughly tested so that most scientists agree on it. It can be changed if new information is found. Theory is different from a working hypothesis, which is a theory that hasn't been fully tested; that is, a hypothesis is an unproven theory.
A physical law or scientific law is a scientific generalization based on empirical observations of physical behaviour, often over many years. They describe observable phenomena and patterns and can be used to make predictions. Empirical laws are typically conclusions based on repeated scientific experiments and simple observations, over many years, and which have become accepted universally within the scientific community. The production of a summary description of our environment in the form of such laws is a fundamental aim of science.