What is the difference between KPI and Scorecard with respect to SharePoint 2010?
A simple answer for your question,
A Key Performance Indicator (KPI) is a visual cue that communicates the amount of progress made toward a goal.
A scorecard is a type of report that displays a collection of key performance indicators (KPIs) together with performance targets for each KPI.
a scorecard is NOT a collection of KPIs as suggested here but rather threasholds defined regarding the KPIs.
If for example. I have a KPI of the number of products sold by a vendor over a month. A scorecard can be defined as 100 products sold and so if this KPI for the last month is 90 then the related scorecard will indicate a negaive alerts (e.g. red icon).