This was originally used for looking at the economy as a whole, but more recently has been applied to firms. Indicators are things that can be measured that ca be used to indicate what has happened or predict what will happen.

As you would expect leading indicators are those that occur before an event and are used as a predictive tool. Lagging indicators follow an event and can provide an accurate snapshot in time.

In terms of the balanced scorecard they are often know as outcome measures for lagging, and performance drivers for leaders. Examples are:

Leading (Predictive):
Time with customer
Product development cycle time
Customer response time
Performance processes (6 sigma)

Lagging (Snapshots in time):
Return on Investment
Revenue
Customer retention rate
Customer satisfaction

NB. It is wort noting that some measures can go in both areas depending on what you are measuring.