Mass is a fundamental property of the Catholic Church - a measurement of how much...
psst - you were supposed to do a physics writeup
Ok... Take 2...
Mass is one of the fundamental properties of matter. Mass is a measure of the amount of matter in an object or a measure of the amount of inertia it has while in motion (and the amount of force that is necessary to act upon it to change this). Unless you have been in space or diving where the effects of gravity are gone or reduced, you are familiar with the weight of something rather than its mass. The weight of an object is the force of gravity acting upon the mass.
Before going too far, people are often confused with the units of mass and weight - especially between the metric world and the English units world.
On Earth, something that weighs 100 pounds has a mass of 3.11 slugs in English units. This something also weighs 444.80 newtons, and has a mass of 45.38 kg.
(Theoretical physics follows)
While we 'understand' to some very sketchy level with lots of hand-waving and funny symbols on the chalk board what gravitation is, and how quarks in a proton stick together and protons stick together in an atom, and energy is transmitted, the 'cause' and 'reason' for mass still confuses physicists to a great deal more than other facts of the universe.
The Standard Model in physics works on established masses (energies) of fundamental particles. From this, the mass of everything up can be understood. However, the Standard Model also says that there is another part to the universe that has not been observed - a field that is (almost) indistinguishable from empty space and permeates all space. It is believed that when a particle interacts with this field it acquires mass. This field is named after Peter Higgs of the University of Edinburgh, Scotland.
The best analogy of how this Higgs field works can be thought of as...
Picture a noder gathering where all the noders are uniformly
spread about (the Higgs field). Nate starts walking through the area and noders are all clustering around him - as he moves, the closest people cluster around him. This clustering is 'mass' - it makes it harder for Nate to move around he has some momentum in the group - its harder to stop this mob from moving, and once stopped it is harder to get it moving again.
The Higgs particle would appear in noder-Higgs field as a rumor where suddenly there is a cluster of noders whispering to each other, but no actual person moving through. This whispering cluster moves through the area and is the same type of thing expected to be seen as the Higgs boson would be a clump in the Higgs field.
While this has not been directly observed (only hints of it have been seen) there are two significant pieces of evidence that the Higgs boson does exist. Quantum theory predicts its existence along with that all particles spend some time as combinations of all other particles - including the Higgs boson. This changes how they interact in measurable ways that have been well verified. Other studies have shown that theories for how other particles act only are consistent if the the Higgs boson exists. However, these are only indirect results and not proofs. However, in the last days of the Large Electron Positron collider at CERN, there was a particle with a mass of about 115 GeV (180 proton masses) that had the characteristics of the Higgs boson. This was not certain, but it was "enhancement above background of less than three standard deviations" (and better than two). The odds that this finding was noise is 1 in 250 (though still too uncertainty to be claimed as a discovery - that needs an odds of error less than 1 in 1,000,000).
The question of what causes mass is still open.
http://www.p-i-a.com/Magazine/Issue10/Physics_10.htm
http://www.hep.ucl.ac.uk/~djm/higgsa.html
http://pdg.lbl.gov/atlas/etours_physics/etours_physics09.html