Hi Ashvini,
Sequential phases are conducted sequentially rather than concurrently. Thus, you are conducting one phase to completion before starting the next phase. For example, phase 1 is started and completed, then phase 2 is started and completed, etc.
Iterative phases are conducted iteratively. Thus, you may bounce back and forth between phases. For example, you might complete phase 1 and then iterate between phases 2 and 3 before moving on to phase 4. In this scenario, you may cycle through phases 2 and 3 several times before moving on to phase 4.
Overlapping phases are similar to sequential phases except that a phase might not be completed when the following phase starts. For example, you might start phase 1 on January 1st and phase 2 on March 1st, and then complete phase 1 on June 1st and phase 2 on August 1st, etc.