By definition, the sequester is a series of automatic spending cuts by the federal government that take place across the board. Some cuts will be to the military and defense, while some will be for "discretionary" or social programs such as Medicaid and Social Security. These cuts are designed to reign in the ballooning federal deficit (now at approximately $16 trillion).
The problem: Republicans and Democrats cannot agree on how to reduce the deficit. Ironically, the sequester was never supposed to happen. President Obama signed it into law under the guise that the cuts would be so painful and catastrophic that Congress would never let it happen.
So much for good planning.