Once equlibrium has been reached, the reaction continues to occur, but with equal rate in opposite directions. This results in constant macroscopic properties and it "appears" that the reaction has stopped. But on the molecular level change continues. That is why the condition is sometimes called "dynamic equilibrium".

The relative amounts of products and reactants present at equilibrium under specified conditions are an indicator of the extent of reaction (i.e., "how far" things went before equibrium was achieved). It has been found experimentally that a certain ratio for the concentrations of reactants and products will be constant for fixed conditions in a particular reaction system.

Generally the larger K_eq, the more spontaneous a reaction is and the more product there will be at equilibrium.

Comparing equilibrium constants for different reactions, however, is tricky business. The law of mass action describes how equilibrium constants are written from balanced equations.