How does a dss typically differ from an eis?

Answer;
An EIS requires data from external sources to support unstructured decisions, whereas a DSS typically uses internal sources to support semi-structured decisions.

Explanation; 
EIS differ from a DSS in that;
-It supports top management
-It uses a lot more external data and makes long term strategic decisions which are not often repeated
-Uses a lot more subjective data. 
DSS (decision support system) , it models information using OLAP, which supports managers and business professionals during decision making process at managerial level. EIS is a specialized DS that supports senior level executives and unstructured, long term, non routine decisions requiring judgement, evaluation and insight.