There are 4 grading services: PCGS, NGC, ANACS, and ICG. The others are companies which enclose your coins in plastic and put a grade on that plastic, a grade which is correct about as often and for the same reason as a broken clock is correct: twice a day.
As of right now, PCGS-slabbed coins bring more at market than the others in almost every category, except the real rarities whose quality outshines the slab.