LAG and LEAD were introduced in SQL Server 2012. They require an OVER clause, but it can only specify PARTITION BY and ORDER BY. No ROWS / RANGE specification for a window frame. Which makes them the stand out as unusual in this series.
By default, they return a value from the last row before the current row, or from the first row after the current row, based on the specified sort order and while observing the specified partition boundaries. But there are two optional parameters, an offset to specify that you want, for instance, the third-last row or the second-next row. And the default parameter specifies a value to be used instead of NULL when the indicated row falls outside of the partition.
Click through to see what the plans look like, as well as how very welcome though potentially performance-impacting changes in SQL Server 2022 have affected this.