A filling involves first, the removal of the decayed part of the tooth surface and then, filling it with a highly durable and esthetic tooth colored material. It allows for a natural and pleasing smile.
Sealants or “dental sealants” are a see-through plastic coating material placed on the grooves of the chewing surface of the molars. It prevents future tooth decay and discoloration.
Here's the American Dental Association's description of fillings and sealants:
"Thanks to advances in modern dental materials and techniques, dentists have more ways to create pleasing, natural-looking smiles. Dental researchers are continuing their often decades-long work developing materials, such as ceramics and polymer compounds that look more like natural teeth. As a result, dentists and patients today have several choices when it comes to selecting materials to repair missing, worn, damaged or decayed teeth. These new materials have not eliminated the usefulness of more traditional dental materials, such as gold, base metal alloys and dental amalgam. That's because the strength and durability of traditional dental materials continue to make them useful for situations, such as fillings in the back teeth where chewing forces are greatest."
"A sealant is a plastic material that is usually applied to the chewing surfaces of the back teeth-premolars and molars. This plastic resin bonds into the depressions and grooves (pits and fissures) of the chewing surfaces of back teeth. The sealant acts as a barrier, protecting enamel from plaque and acids. Thorough brushing and flossing help remove food particles and plaque from smooth surfaces of teeth. But toothbrush bristles cannot reach all the way into the depressions and grooves to extract food and plaque. Sealants protect these vulnerable areas by "sealing out" plaque and food."
For more details, visit the American Dental Association (ADA).