Salesforce Object Search Language (SOSL)

Data stored in the Force.com database is automatically indexed to support both structured and unstructured queries. SOQL is the language for structured queries, allowing records from a single object and its related objects to be retrieved with precise, per-field filter conditions. SOSL is a full-text search language for unstructured queries. It begins by looking across multiple fields and multiple objects for one or more search keywords, and then applies an optional SOQL-like filter on each object to refine the results.

To decide which query language to use, consider the scope of the query. If the query spans multiple unrelated objects, SOSL is the only practical choice. If the query searches for words within many string fields, it can probably be expressed more concisely in SOSL than SOQL. Use SOQL for queries on a single object with filters on various data types.