In software, "top down" design develops a master plan for a program, then splits it up into a small number of large tasks, then breaks them up into smaller tasks, and so on, until all the tasks are identified. Then programming may begin to implement the functions, procedures, and objects needed.

In database design, it identifies all the data and their structures, before any tables and fields are created.

For the Information Architect and in Content Management, "top down" refers to identifying the content in the context of how it is used. User Research is the starting point for User Centered Design?. The emphasis is on Usability and the User Experience. Only then can mockups of the content (a Wire Frame, Blue Print, etc.) be prepared for User Testing.

The opposite Bottom Up design looks at the details first to handle content as it is identified.

References:
BigIA, IAWiki
Wikipedia Top-down