What is the difference between a lead and a lag in scheduling?

A lead is an acceleration of the successor activity, allowing it to start before the predecessor is fully complete. For example, a finish-to-start relationship with a ten-day lead means the successor can start ten days before the predecessor finishes. A lag is a mandatory waiting time between activities. For example, a finish-to-start relationship with a five-day lag means the successor must wait five days after the predecessor finishes before it can start, such as waiting for concrete to cure before building on it. Leads compress the schedule while lags extend it.