In the sense in which you are using them there is very little difference between the two except that "job" is less formal and is slightly more likely to be used to refer less secure, lower status, and less career orientated positions. For instance "he had a job as a circuit judge" wouod be perfectly correct but "he was employed as a circuit judge" would be more normal.
However both words have more than one definition.
In addition to being a synonym for employment job can mean the responsibilities someone has as part of their employment (his job is to drive the refuse truck, her job is to perform open-heart surgery); a particular task, whether paid or not (repainting a whole office block is a big job, doing the laundry for a family with six children is a big job) and there are a few less common meanings.
Employment also has more than one meaning, in addition to being a synonym for job it can mean the action of giving paid work to someone (The employment of a gardener gave her more time to write) or the utilisation of something (full employment of the machinery will increase production).
When you are talking specifically about someone working for an organisation or individual and getting money for the work job and employment are very close in meaning, but the other uses of the words diverge considerably.