Microsoft Search Server is a single search solution that allows searching across multiple content sources like SharePoint, file shares, public folders, databases and websites. It provides features like relevance tuning, continuous indexing, federated search connectors and security trimmed results. Search Server Express has limitations like a 4GB database size limit but is good for environments with under 1 million documents. Federated search allows querying external search engines while Search Server integration adds search functionality to SharePoint sites and other applications.