An "Algorithmic Composition program" is software that has the capacity to create a complete musical arrangement on your behalf. Such a program typically allows you to hear (ie, play) the arrangement that it creates, and even edit the arrangement somewhat. Therefore, such a program is a specialized breed of sequencer software.
The program composes all of the individual musical parts (for example, drums, bass guitar, piano, etc, parts). You need specify only what chord changes are in the arrangement (ie, what chord plays upon which musical bars and beats), what instrumentation you would like, and what "style" of music you wish.
The program usually offers a variety rock, jazz, country, waltz, rap, latin, etc, "styles" of music.