It is very much like ordinary matter, but it has the important distinction that the particle has exactly the opposite charge. The anti-matter electron (the positron) has a positive charge but has the same mass and spin as the ordinary electron. The anti-matter proton has a negative charge, not a positive one. But an anti-neutron has a zero charge, so why is it considered an antiparticle? Because the proton and neutron are made from matter quarks, and we can just as easily assemble them from quarks made from anti-matter. Here's how to do it. For neutrons and protons there are two types of quarks that compose them; Up quarks (U) and Down quarks (D). A proton is made from two up quarks and one down quark; UUD and a neutron from a UDD combination. For the proton, its anti-proton consists of two anti-U quarks and one anti-D quark, which give a net charge of -1. A similar swap for the anti-neutron gives two anti-D and one anti-U quarks, which have a net charge of zero. So, anti-matter is more than just the reversal of the charge for a particle. If the particle is composite, like the proton and neutron, you have to build them from anti-quarks. Matter and anti-matter don't obey exactly the same laws of physics. The K (and B) mesons, for example, seem to decay in slightly different ways between the two types of matter. The decays tend to favor about 0.2% more matter than anti-matter in the end products. Cosmologists are very excited by this old discovery because it helps to explain why our universe is totally dominated by matter. Meanwhile, new experiments are testing the idea that matter and anti-matter are accelerated by gravity at exactly the same rates - a key prediction by General Relativity. Its' a tremendously difficult experiment to carry out, but if any differences are uncovered it will be a problem for General Relativity and all the ideas that have flowed from this theory in the last 90 years.
