what is "no post" and how is it different from no boot?

Answer 1

As Mortis answered POST is the testing and boot is the initialization process. You could say that the post is part of the boot process. By selecting No Post at the bios your computer will initialize or boot faster as the testing step is skipped. The post step however could help you identify hardware problems like defective memory.

Answer 2

Power-on self-test (POST) is the common term for a computer's, router's or printer's pre-boot sequence. Post is the preliminary tests run by a piece of hardware to verify that it is working correctly, boot is the initialization of the operating software for the hardware.