A DSS - or decision support system - is designed to provide effective analysis relevant to specific situations while an MIS - management information system - is designed for the efficient processing of data or information. As such, they are generally used by different people in different situations.
DSS are used to provide support to analysts and decision makers within an organization that are relevant to a specific problem or situation and make an evaluation of various different outcomes. They are used by managers and other decision-makers in both unstructured and semi-structured situations and largely in ad-hoc situations although it may involve repetitive decisions. A DSS is used in situations where individual managerial judgment is required.
An MIS provides routine information typically on an ongoing basis in a standardized format. Reports may be run routinely or be provided on demand. While an MIS can be used to solve standardized and routine problems using specified criteria and the data from an MIS forms the base pool of information for a DSS, MIS are also used for routine functions such as production control and monitoring, forecasting, human resources management, financial analysis and research. Because of the broad nature of the information presented, an MIS might be used by lower, middle or upper level management.
While an MIS simply gathers data, a DSS manipulates that data and helps to develop tools that aid in the decision-making process.