In most reports, a discussion of the data will be mixed in with the results. The discussion may add an interpretation to the data and relate the present data to the theory or previous data. Sometimes the discussion is long enough and involved enough that a separate section for Discussion of Results is needed. The need for a separate section is a subjective decision based on whether the information flows best when mixed together or separated. These are two distinctly different elements of a report which are almost always grouped together in the same section.
Conclusions are statements which can be made based on the information given in the report.
The theory section often introduces questions which will be answered by the experiment. The conclusions are the answers to those questions. It is absolutely essential that the conclusions be supported by data and discussion in the text. Conclusions should not be pulled out of thin air, gut feeling, or previous experience. Sometimes the discussion or analysis of the data leads to conclusions which may be somewhat disputable.