Completions: The total number of completions for the requested goal number
Total Completions: The total number of completions for all goals defined for your profile.
Conversion Rate: (ga:goal(n)Completions / ga:visits) * 100
Total Conversion Rate: (ga:goalCompletionsAll / ga:visits) * 100
Reference: http://code.google.com/apis/analytics/docs/gdata/dimsmets/goalconversions.html