Your needs & your website's needs will largely determine what you want from a CMS, it is important to consider the following features however:
Web page templates (with the ability to update templates if needed): the templates should conform to current recognized web standards for Web technologies and accessibility to ensure maximum compatibility across different browsers and platforms.
Security and access rights for different types of user: e.g. only those with the correct authorization should be able to publish material to the live site once its content has been approved.
Integration with existing systems: for example if you already have a suitable database could this be used? Do you need the CMS to convert your existing Word documents to HTML?
Non technical content creation and publishing: perhaps your content authors should not be required to use HTML or other technical knowledge to create Web pages.
Facility to hold, edit and format text, and to link to and add other digital content such as images, audio and video to a Web page easily.
Ability to record information about the content (metadata) such as author, when it was updated, when it needs to be reviewed, etc., and the means to report on this information.
If the CMS is likely to have many simultaneous users, features such as record locking will be important to ensure that clashing changes are prevented.
Ability to add, remove and archive content: a CMS could also offer the means to specify when a page should be put live on a site, and when it should be taken down (e.g. for press releases and news items).
Link management: so that when a new page is added or a page is deleted, links to or from other pages in the site are automatically created or removed. You may also require a facility to be notified automatically if there are any problems with links to external sites Tools to allow the CMS administrators to create online surveys and polls and to easily collect, store and publish the results.
Ability to run on your existing equipment.
Content management systems can range from fairly simple to those with highly sophisticated workflow management processes built-in. Some systems may require users to know a few basic HTML tags to format text while others will only require simple word processing skills.
The more complicated your needs are, the more expensive the CMS is likely to be. Before identifying your specific requirements, it is important to:
Determine which goals of your organization will be achieved by implementing a CMS (have a business case for getting one)
Involve all the potential stakeholders in the process of drawing up your requirements; survey what information needs to be published via the CMS as this will influence the functionality you need.
Once you've decided on your requirements, specify them in plain language & in as much detail as possible giving examples wherever you can. This will help potential suppliers understand your needs better.