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_DateNext_Start_DatePrev_Start_Date
20/02/201122/02/2011NULL
22/02/201123/02/201120/02/2011
23/02/2011NULL22/02/2011