Different clients will have different levels of documentation on what they want to be built. You need to be ready to do a Spec Review for any one of the following possible cases:
There are 5 levels/types of Specifications:
Run from this
or
Verify they have a really hefty bank account!
This will read like a wish list with no details and many unanswered questions. High-Level requirements are very vague and open to many interpretations.
The details have been fleshed out and allow developers to write Functional and Technical Specifications.
We need a login page for northwind.com
Must match existing site look and feel
Username is already in the Users table in the ABC database (SQL Server)
Password should be at least 8 characters
.NET is already used for the existing site so that is what this should use of course
Figure: Detailed Requirements have more of the details you want
This will include detailed mock-ups for the UI, use cases/user stories and might be at a level to allow for fixed price quoting on the project. Functional Specifications go into more detail about the user interface and interactions in the system.
We need a login page for northwind.com
Must match existing site look and feel
Users table must be defined and added to the ABC database (SQL Server 2008)
Username consists of user first initial and first 7 characters of the last name
Password should be at least 8 characters
Site uses .NET 4 and this interface must be added to existing project
This is the layout for the login interface
A red asterisk (*) should be displayed if a value is left blank and Submit is pressed
This is the blueprint for the application. There should be no unanswered questions and should allow for a fixed-price quote.
We need a login page for northwind.com
Must match existing site look and feel
Users table must be defined and added to the ABC database (SQL Server 2008)
Username consists of user first initial and first 7 characters of the last name
Password should be at least 8 characters
Site uses .NET 4 and this interface must be added to existing project
Define the data model explicitly

Must work with IE7, IE8, IE9, and FF3
Must display correctly at 1024x768 resolution
Must support ANSI characters for Username and Password
Will not support mobile browsers
Will not be tested with localization (assumes en-us local on US versions of software)
SQL Membership provider will be leveraged