PL/SQL What is the difference between Lead and Lag functions

What is the difference between Lead and Lag functions

The Lead function returns values from the next row, while the Lag function returns values from a previous row.

Example

Select start_date, 
lead (start_date,1) over (ORDER BY start_date) AS next_start_date
lag (start_date,1) over (ORDER BY start_date) AS prev_start_date
From contracts
Where customer_id = 760;
Start_Date Next_Start_Date Prev_Start_Date
20/02/2011 22/02/2011 NULL
22/02/2011 23/02/2011 20/02/2011
23/02/2011 NULL 22/02/2011