what questions ( as a job seeker ) do you ask the employer in a job interview?

There are many standard questions that you can ask and dependent on the type of job applying for.
If the question hasn't been answered during the interview - useful questions to ask are: "how did the job/vacancy come about?" You need to know why there is a vacancy.
Definitely ask a question about the companies plans for the future as they relate to the job - this shows that you are interested in staying for the long term.
Ask a question for which the interviewer will remember you.
Something that puts in their mind that you're serious and interested in the job.

Your basic question should be what does the job in hand demand. What is the size of the team you are to handle or what is expected of you in terms of targets.

If the job is more analytical in nature, then ask how the organisation will be fitting you into their scheme of things and how your skills would be utilised. DO NOT ask about the remuneration as HR would be helping you out with that.