We propose solutions for several issues which arose over time as the Web and Information Retrieval evolved. By performing syntactic analyses we create a model for Web query reformulation processes and provide interesting insights into users' Web behavioral patterns. We choose to follow Query Expansion patterns and propose to personalize Web queries by expanding them with terms collected from each user's Personal Information Repository. We then study the usefulness of collaborative tagging for identifying which characteristics of the objects are predominantly described and what kinds of tags are employed across multiple domains and resource types. By performing a similar analysis on user queries we identify the gaps between the tag space and the querying vocabulary - and bridge the identified gaps. Moreover, our algorithms are also able to exploit the social information produced by users, for classifying media into different event categories. This allows browsing and organization of picture collections in a natural way, by events and opens new possibilities for multimedia retrieval.