Application integration involves connecting different enterprise applications and data sources to allow for sharing of business data and processes. The key goals are to integrate legacy systems, share distributed content repositories, simplify processes, and provide a single user experience. When planning integration, it is important to understand existing systems, scope what will be integrated, and choose a methodology like mediation, federation, or a hybrid approach using techniques like web services and standard interfaces. The benefits include centralized access and discovery, while the main disadvantage is high initial costs.