ALV means ABAP List View .It is used in t-code 
SE38,Excautable program . more than  255 lines of 
characters in length we use ALV reports .
with function modules 

Reuse _ALV_LIST_DISPLAY
Reuse_ALV_GRID_DISPLAY

but SLIS is created in se11-type pools to work in ALV 
reports

SLIS_ALV_FIELDDATACAT.
SLIS_ALV_LAYOUT
SLIS_ALV_EVENTS.