An operational data store (or "ODS") is a database designed to integrate data from multiple sources to make analysis and reporting easier. Because the data originates from multiple sources, the integration often involves cleaning, resolving redundancy and checking against business rules for integrity. An ODS is usually designed to contain low level or atomic (indivisible) data (such as transactions and prices) with limited history that is captured "real time" or "near real time" as opposed to the much greater volumes of data stored in the Data warehouse generally on a less frequent basis.

An ODS is designed for relatively simple queries on small amounts of data (such as finding the status of a customer order), rather than the complex queries on large amounts of data typical of the data warehouse.

Refreshment Speed of ODS:

• A Class I ODS is used when the information must be very accurate and up-to-date at all times, e.g., the customer service representative talking to a customer must see his most current information no matter where it was initially entered or changed.
• A Class II ODS is a little more relaxed, using store and forward techniques for data update rather than performing synchronous updates. A class II ODS receives updates every half hour or hour. The customers summary web purchases may be updated into the customer ODS every 30 minutes.
• A Class III ODS is typically updated in batches, most often on a daily basis. A customers product preferences may be updated into the ODS only once every day or so. Information currency requirements are not nearly as robust when organizations build a class III ODS. Because the product preferences are used to understand cross-sell recommendations, and because they do not change that frequently, class III updates work well
• The fourth type of ODS, Class IV, is a special case where information provided to the ODS comes not only from the operational systems but also from the data warehouse or specific data marts. The information from the data warehouse or data mart is transferred into the ODS only periodically, usually in a scheduled fashion.

References:

Wikipedia

Active Data Warehousing — The Ultimate Fulfillment of the Operational Data Store (www.IntelSols.com)