These two terms are often confused and even experience potters sometimes use them incorrectly. A much more detailed explanation of these terms can be found at wikipedia.org
Maiolica
Maiolica is a technique of applying or brushing pigments on raw or unfired glaze. It was developed by the Italians, Spanish and Persians in the 15th and 16th centuries. The base glaze is traditionally glossy and is whitened with tin oxide. It is often lead-based; although,today, many maiolica potters are using lead-free glazes. The pot is normally fired to earthenware temperatures. It is a technique widely used by potters who like to use their pots as a "canvas" for drawing or painting.
Majolica
Majolica is a late 19th-century English pottery with shiny, colorful glazes. The clay body fires to a light straw color and the pots were heavily modeled to provided areas of glazes highlights and areas where glaze collected more heavily.