How to formulate job descriptions for various posts in our company?

I work as a corporate Manager - HR/ IR with the dealer. Our workforce is around 80 employees.
We have designations as under-
Sales-
Tr. Engg.
Engineer
Marketing officer
Segment Manager - like Industry,CBS,WTP etc.
Area Manger
Regional Manager - As we have three Branches-Pune/Thane/Goa
Service
Technician,
Senior Technician
Manager
Accounts - Executive,
Officer, Manager etc.
Purchase -Assistant
HR Assistant
Commercial Assistant/ Executive
Our company is a dealer of one of the international PUMP Companies and the owner, my boss, wants a Job description of all the catagories.
Kindly help me with the same.

Job Description for any unit has to come from within the organisation. For example, to write a proper/ adaptable/ expected Role and Job responsibilities, can be done by a person who is his immediate Manager to whom the employee will be reporting. Then his peer group members and last but not the least, his immediate subordinate, about their expectation. You could ask the concerned employees individually to write the respective job descriptions as an exercise in a training session. Thereafter, if you compile all the information so gathered, you will find a 'best, adaptable roles and responsibilities' for the particular staff.
This exercise needs to be done internally. No other firm's roles and responsibilities of staff can be made applicable/ adaptable to that of your organisation as the same would create lot of barriers for the staff to perform in their present service conditions.
I hope my response is clear to you. You may either do it in your training session or engage a trainer to prepare a draft based on the similar exercise and compilation of data.

Thank you for your valuable comments - they are very helpful.