The contractor if someone or an organisation that contracted in by the organisation directly to carry out a specific job or task, for a predetermined fee or contract sum. They will also bring in on occasions sub-contract specialists in different areas to assist in completing the project defined. Contractors differ from employees in that they pay no National Insurance, PAYE etc. They generally do not offer any advice to the organisation; they are generally there to complete the task or service in conjunction with the employees.
Whereas a consultant, his brought into the organisation as an external professional advisory source of information. Procurement Consultants have no direct authority to make decisions and are hired on a temporary basis. Consultants would normally work on a daily rate plus expenses, sometimes if they have targets to meet and may add in a percentage bonus of the savings made.
You can get internal consultants that are employed on a permanent basis (usually by larger organisations i.e. hospitals, police forces, local government etc.), that are kept on board to advise all departments or personnel as and when required. The main difference between a consultant and a contract is one of direction, I consultant will be hired to provide a solution to specific issues, within a budget and resources agreed with the client. Contractors on the other hand normally bridge a gap where staffing shortages are apparent, and a defined role is already set.
There is however consultants who work under contract in an interim role, such as a manager or an executive bringing a combined specialism on a temporary basis.