ALV is Application List viewer.

Sap provides a set of ALV (ABAP LIST VIEWER) function
modules which can be put into use to embellish the output of
a report. This set of ALV functions is used to enhance the
readability and functionality of any report output. Cases
arise in sap when the output of a report contains columns
extending more than 255 characters in length. 

In such cases, this set of ALV functions can help choose
selected columns and arrange the different columns from a
report output and also save different variants for report
display. This is a very efficient tool for dynamically
sorting and arranging the columns from a report output. 

The report output can contain up to 90 columns in the
display with the wide array of display options.

The commonly used ALV functions used for this purpose are;

1. REUSE_ALV_VARIANT_DEFAULT_GET
2. REUSE_ALV_VARIANT_F4
3. REUSE_ALV_VARIANT_EXISTENCE
4. REUSE_ALV_EVENTS_GET
5. REUSE_ALV_COMMENTARY_WRITE
6. REUSE_ALV_FIELDCATALOG_MERGE
7. REUSE_ALV_LIST_DISPLAY
8. REUSE_ALV_GRID_DISPLAY
9. REUSE_ALV_POPUP_TO_SELECT