The Higgs boson is an undiscovered elementary particle, thought to be a vital piece of the closely fitting jigsaw of particle physics. Like all particles, it has wave properties akin to those of ripples on the surface of a pond which has been disturbed; indeed, only when the ripples travel as a well-defined group is it sensible to speak of a particle at all. In quantum language the analogue of the water surface which carries the waves is called a field. Each type of particle has its own corresponding field.
The Higgs field is a particularly simple one - it has the same properties viewed from every direction, and in important respects is indistinguishable from empty space. Thus physicists conceive of the Higgs field as being "switched on", pervading all of space and endowing it with a "grain" like that of a plank of wood. The direction of the grain is undetectable, and only becomes important once the Higgs' interactions with other particles are taken into account. For instance, particles called vector bosons can travel with the grain, in which case they move easily for large distances and may be observed as photons - that is, particles of light that we can see or record using a camera; or against, in which case their effective range is much shorter, and we call them W or Z particles. These play a central role in the physics of nuclear reactions, such as those occurring in the core of the sun.
The Higgs field enables us to view these apparently unrelated phenomena as two sides of the same coin; both may be described in terms of the properties of the same vector bosons. When particles of matter such as electrons or quarks (elementary constituents of protons and neutrons, which in turn constitute the atomic nucleus) travel through the grain, they are constantly flipped "head-over-heels". This forces them to move more slowly than their natural speed, that of light, by making them heavy. We believe the Higgs field responsible for endowing virtually all the matter we know about with mass.
Like most analogies, the wood-grain one is persuasive but flawed: we should think of the grain as defining a direction not in everyday three-dimensional space, but rather in some abstract internal space populated by various kinds of vector boson, electron and quark.
The Higgs' ability to fill space with its mysterious presence makes it a vital component in more ambitious theories of how the Universe burst into existence out of some initial quantum fluctuation, and why the Universe prefers to be filled with matter rather than anti-matter; that is, why there is something rather than nothing. To constrain these ideas more rigorously, and indeed flesh out the whole picture, it is important to find evidence for the Higgs field at first hand - in other words, find the boson. There are unanswered questions: the Higgs' very simplicity and versatility, beloved of theorists, makes it hard to pin down. How many Higgs particles are there? Might it/they be made from still more elementary components? Most crucially, how heavy is it? Our current knowledge can only put its mass roughly between that of an iron atom and three times that of a uranium atom. This is a completely new form of matter about whose nature we still have only vague hints and speculations and its discovery is the most exciting prospect in contemporary particle physics.