When we refer to the post or the position that a person holds, we are referring to their job. We would say that Barack Obama holds the post of President of the US. We would say that that is his position.
Some people get their positions through appointment. A prime minister, for example, might appoint someone else to the post of foreign minister or defense minister. In American English, at least, it would be odd to use appointment and post in similar ways. We would say "his job is foreign minister" or "his post is foreign minister" but we would not typically say "his appointment is foreign minister."
In the link below, you can see an example of the term "post" at the start of the 4th paragraph.