Werner Heisenberg was an eminent German theoretical physicist (5/12/1901 - 1/2/1976). He was awarded the 1932 Nobel Prize in Physics "for the creation of quantum mechanics". He is probably best known for the Uncertainty Principle.
Things get weird, the smaller we go! The principle tells us that there is a fundamental limit to what we can know about the behaviour of quantum particles (the smallest subatomic particles). When we get to a quantum scale, the most we can hope for is to calculate probabilities for where things are and how they will behave.
We cannot measure the position and the momentum of a particle with absolute precision. The more accurately we know one of these values, the less accurately we know the other. Multiplying together the errors in the measurements of these values (the errors are represented by the triangle symbol in front of each property, the Greek letter "delta") has to give a number greater than or equal to half of a constant called "h-bar". This is equal to the reduced Planck constant (usually written as h / 2π). Planck's constant is an important number in quantum theory, it's a way to measure the granularity of the world at its smallest scales.
Among its many counter-intuitive ideas, quantum theory proposed that energy was not continuous, but instead came in discrete packets called quanta. Quantum mechanics differs from classical physics in that energy, momentum, and other quantities are restricted to these discrete packets; and objects have characteristics of both particles and waves (wave-particle duality).