You should write three or four sentences explaining what conclusion you can draw from your analysis of the data.  In your conclusion, you should:

• Restate the objectives.
• Summarize of the method used.
• Restate any major findings. (e.g. "Ohm's Law, V = IR, was validated by our experiment")
• Answer any assigned questions.
  

In addition, I am looking for the following qualities in your conclusion:

• Understanding of the concepts. (Did you really know what you were doing, or were you just following the procedure brainlessly?)
• Professionalism.  ("It was fun!"  "It was stupid."  "I don't get it."  "Clearly we did something wrong." "We had bad equipment." "I learned a lot." are NOT prefessional statements.)
• Uncertainty in your findings.  (e.g. "The value of R was 2.0 +/- 0.1 kOhms.")