Leads: It is the speeding up an activity. There will be situations you will have to schedule a task in such a way that this need to be started before the ending of a previous task.
Lags: It is the lagging/slowing of an activity. If a task need to be delayed, you can say lag. Eg: In construction, you many need to lag painting of sealing, as the ordered ladder will arrive after 10 days only.