The Combine Occupation is the current state of Earth as of 2010. After the events of the Resonance Cascade at the Black Mesa Research Facility, portal storms dotted the landscape of Earth, allowing interdimensional beings to invade and make their home here.
It is the current state of affairs as we know it in the Half-Life 2 Universe.
After years of storms, the Combine (a conglomeration, or 'combination' of many, many races on an interdimensional scale) caught wind of Earth's situation, and launched a full-scale invasion that lasted little more than 7 hours. This is known as the 7-Hour War.
Many of Earth's cities have been invaded and converted to what is called an "Urban Center". The Combine instilled the Overwatch system as a means of military, while Overwatch put into place a Metropolitan Police system known as the Civil Protection. Civil Protection avidly patrol the streets and crush rebellion as they see it.