For instance, Jon Rubenstein was the CEO of Palm before it was acquired by HP. Would it be accurate to call him the former CEO of Palm or should you call him the ex-CEO of Palm? Strictly speaking, both terms would have exactly the same meaning: he once held this position, but he no longer holds this position.
That said, English can be a very confusing language and word choice very much comes into play. If Mary and Henry got divorced, Mary would refer to Henry as her ex-husband. It just sounds odd to say that he is her former husband. Similarly, some may say it’s odd to refer to Bill Clinton as the ex-President of the United States; it’s generally more accepted to say that he is the former President. Both have the same meaning, but one is preferred over the other.
One very important difference to note between ex- and former, though, is that ex- is a prefix while former is a complete word on its own. In this way, ex- has to be connected to the former position with a hyphen. Examples include ex-wife and ex-employee. Colloquially, an “ex” (as its own word) could refer to a former partner, as would be the case with an ex-girlfriend or ex-husband. Former would still go in front of the former position, but it is not connected with a hyphen. He is the former boss of Palm. She is a former Marine.
And then there are all sorts of instances where conventional word choice would lead you one way or the other. If you once lived somewhere, it might be acceptable to call that dwelling your former home, but it doesn’t sound right to call it your ex-home. In like manner, if you once worked somewhere, you could say that they were your former employer (as in, “I was formerly employed at XYZ Company”). Saying that XYZ Company is your ex-employer doesn’t sound right either.
Yes, English can be very confusing. The difference between a job and a career can a simple matter of semantics, but some word choices have no real rhyme or reason behind them. You just learn by reading and listening to native English speakers.