Antimatter is matter that is in all respects, the opposite to 'ordinary' matter.
So antimatter has all its charges opposite to ordinary matter. Antimatter doesn't mean the opposite of matter in terms of mass, which is where people often go wrong. Matter and anti-matter particles interact through the same forces as matter interacts with matter. It's thought that matter and antimatter were created in almost equal amounts in the big bang and as time progressed, the antimatter and matter collided and annihilated each other, to leave the left over bits of matter that make up the stars and galaxies today.