There are actually 3 checkpoints in the cell cycle:
The first checkpoint is located at the end of the cell cycle's G1 phase, just before entry into S phase, making the key decision of whether the cell should divide, delay division, or enter a resting stage.
The second checkpoint is located at the end of G2 phase, triggering the start of the M phase (mitotic phase). In order for this checkpoint to be passed, the cell has to check a number of factors to ensure the cell is ready for mitosis.
The third is the spindle checkpoint to be sure there are equal number of chromosomes on either side, Down's syndrome is caused by a problem here.