Philip Halpert is the second child of Jim and Pam. He was born off screen and wasn't seen on air until a few months after his birth. He was assumed to be born around December 5th or so based on Pam's online blog.
Philip is named after Pam's grandfather. He is the younger brother of Cece Halpert who often calls him Pull Up instead of Philip.
The character of Philip was brought on the show because actress Jenna Fischer got pregnant.
Philip is also the name of Angela's baby that she had on the episode Jury Duty.