The Google Geocoding API is a webservice for the Google Maps API. The Geocoding API provides a direct way to access a geocoder via an HTTP request, though it must be used in conjunction with Google Maps. Additionally, the service allows you to perform the converse operation (turning coordinates into addresses); this process is known as "reverse geocoding." Also, you can choose to limit your search to certain bounded areas (Viewport Biasing) or regions (Region Biasing).
Almost every app that uses a mapping API needs latitude/longitude points. To get geocodes you need to convert an address to a point on the earth. That's where you'll need one of the 54 geocoding APIs in our directory. This post covers seven free geocoders and compares them on several aspects.