Complex sentence: Rus-M is a launcher design which was intended to become Russia's main launch vehicle for manned spaceflight after 2018, and an integral part of the Prospective Piloted Transport System which included a new manned spacecraft being developed to replace the Soyuz.
Split into simpler sentences: Rus-M was a planned rocket which was intended to become Russia's main launch vehicle for manned spaceflight. It was to be the launcher for the Prospective Piloted Transport System, a new manned spacecraft which is to replace the Soyuz.