The word מלאכה is the feminine form of the word מלאך. Melacha is the tangible result of using and forming material for a specific purpose. With the universe’s formation complete, it receives the title of God’s מלאכה.
The word שבת means more than rest. It is a halt in activity because the activity has come to its desired end and purpose. With all of creation in place and free willed-Man given the world with the conditions that God had set forth, all new creation halted because there was nothing more to add.
Shabbos Beraishis itself was the last thing put in place for the education of Mankind. It was blessed, endowing it with the ability to continuously foster an awareness of God in the world. It was made holy, given a unique place, to endure forever.
God blessed this day and made it holy because this is a day for Man’s education. It is a time for Man to dwell on God the Creator and the purpose of all of Man and creation. All of creation was designed to lead up to this Shabbos education. This education of Man will gradually, in the course of human history, win over Mankind to become the messenger of God in this world he was intended to become – making use of and uplifting all the universe in service to God.
Beraishis 2:2-3, Pages 43-47