Specifications are part of a contract but often are a separate document from the main form of contract. Is there a simple rule of thumb about what goes in each document?

Contract is a legally binding agreement between two or more parties who agree to buy or sell goods and services from one another. There are many different types of contracts. The three most common contract types include: Fixed-Price contracts, Cost-plus contracts, Time and Materials contracts. 

They maybe combine different types of contracts to create one that hits all the high notes of their business exchange. Some business exchanges include a range of products and services, such as labor and equipment.

The contract defines how the work will be managed and what the responsibilities of each party to the contract are. Whereas the specifications define the works themselves and what exactly is to be undertaken. So the contract sets out "who" is responsible for the various aspects of work and how their management is administered, whereas the specification details exactly "what" is to be built and the required level of quality.