"Premier Cru" and "Grand Cru" refer to the grapes used, and classify the wine based on vineyard type. In theory, the best vineyards grow the best grapes, which in turn make the best Champagne. Grand Cru vineyards are at the top of the scale and Premier Cru is just below, representing high-quality vineyards.

If there is no Cru designation, that doesn't necessarily mean it's not good; it just means that the wine bosses haven't gone in there and certified the vineyard yet.