Alkalies (pl. noun) are non-coloring metallic oxides that react with acidic ingredients in the glaze recipe when heated and thereby produce silicates (glass). Or in other words, alkalies are fluxes.
"Strong" alkalies are lithium oxide (Li2O), Sodium oxide (Na2O), and potassium oxide (K2O).
Alkaline earths, "weak" alkalies, are sometimes also referred to as "bases" by those with a strong chemistry background. Some of them do impart color or opacity and are not often used as the main flux in a glaze. Alkaline earths include: Beryllium oxide (BeO), Magnesium oxide (MgO), Calcium oxide (CaO), Strontium oxide (SrO), Barium oxide (BaO), Zinc oxide (ZnO), Cadmium oxide (CdO), Lead oxide (PbO), and Bismuth oxide (Bi2O3).
Something to be aware of is that the "strong" and "weak" designations do not refer to their fluxing action, but to their solubility in water. For example, lead oxide is a very strong flux but is not very soluble in water.
Feldspars are a type of rock which can contain high concentrations of alkalies; for example, soda 'spars contain high levels of sodium oxide and potash 'spars contain high levels of potassium oxide.