Incinerators are colossal machines built by Buy n Large corporation. Incinerators were used to get rid of the trash in "Operation Cleanup". They are city block-wide sized and move on big tank treads. Each incinerator has a separate crane unit that is able to slice down the trash towers (built by WALL•Es) into digestible sections for the incineration.
The incinerators and their cranes were seen towering over the city buildings when WALL•E was heading home from work. All of the incinerators and their cranes were standing still because they had been deactivated long time ago.